Use Preferences Save Serial Number instead of flag

* new class `PrefsVersion` in prefs.py.  A singleton `prefsSaved` (note
   case) is created.
 * When new preferences are added and require defaults on first run the
   code should use:

     `if prefsVersion.shouldSetDefaults(<prior release version>, [<optional old test>]):`

   to check if defaults should be set in preferences.  So if prior release
   was '3.4.6.0' and you've added a new preference with defaults you should
   call this with '3.4.6.0' as the first argument.
   The <optional old test> is really only for historical purposes, as a
   fallback in case no 'PrefsVersion' has yet been set in the user's
   Registry/settings file.
 * Any code that adds such a new preference **MUST** make changes to the
   `versions` dictionary in the PrefsVersion class.
     1. Add the predicted next version to the dictionary, with number one
        higher than 'current'
     2. Set 'current' equal to that new value.
   Obviously if other post-last-release code has already done this then you
   don't need to.

   Failure to update the versions dictionary in this manner will lead to an
   Exception being raised, and the code the preferences are for failing
   (i.e. EDDN means no EDDN tab on Settings).

Closes #407

plugins/eddn.py

@@ -14,6 +14,8 @@ import tkinter as tk
 from ttkHyperlinkLabel import HyperlinkLabel
 import myNotebook as nb
 
+from prefs import prefsVersion
+
 if sys.platform != 'win32':
     from fcntl import lockf, LOCK_EX, LOCK_NB
 
@@ -347,7 +349,7 @@ def plugin_prefs(parent, cmdr, is_beta):
     BUTTONX = 12	# indent Checkbuttons and Radiobuttons
     PADY = 2		# close spacing
 
-    if not config.getint('PrefsDidSave'):
+    if prefsVersion.shouldSetDefaults('0.0.0.0', not bool(config.getint('output'))):
         output = (config.OUT_MKT_EDDN | config.OUT_SYS_EDDN)	# default settings
     else:
         output = config.getint('output')

prefs.py

@@ -10,7 +10,7 @@ from tkinter import colorchooser as tkColorChooser
 from ttkHyperlinkLabel import HyperlinkLabel
 import myNotebook as nb
 
-from config import applongname, config
+from config import applongname, config, appversion
 from hotkey import hotkeymgr
 from l10n import Translations
 from monitor import monitor
@@ -18,6 +18,70 @@ from theme import theme
 
 import plug
 
+###########################################################################
+# Versioned preferences, so we know whether to set an 'on' default on
+# 'new' preferences, or not.
+###########################################################################
+
+
+class PrefsVersion(object):
+    versions = {
+        '0.0.0.0': 1,
+        '1.0.0.0': 2,
+        '3.4.6.0': 3,
+        '3.5.1.0': 4,
+        # Only add new versions that add new Preferences
+        'current': 4,  # Should always match the last specific version, but only increment after you've added the new version.  Guess at it if anticipating a new version.
+    }
+
+    def __init__(self):
+        return
+
+    def stringToSerial(self, versionStr: str) -> int:
+        """
+        Convert a version string into a preferences version serial number.
+
+        If the version string isn't known returns the 'current' (latest) serial number.
+
+        :param versionStr:
+        :return int:
+        """
+        if versionStr in self.versions:
+            return self.versions[versionStr]
+
+        return self.versions['current']
+
+    ###########################################################################
+    # Should defaults be set, given the settings were added after 'addedAfter' ?
+    #
+    # config.get('PrefsVersion') is the version preferences we last saved for
+    ###########################################################################
+    def shouldSetDefaults(self, addedAfter: str, oldTest : bool=True) -> bool:
+        pv = config.getint('PrefsVersion')
+        # If no PrefsVersion yet exists then return oldTest
+        if not pv:
+            return oldTest
+
+        # Convert addedAfter to a version serial number
+        if addedAfter not in self.versions:
+            # Assume it was added at the start
+            aa = 1
+        else:
+            aa = self.versions[addedAfter]
+            # Sanity check, if something was added after then current should be greater
+            if aa >= self.versions['current']:
+                raise Exception('ERROR: Call to prefs.py:PrefsVersion.shouldSetDefaults() with "addedAfter" >= current latest in "versions" table.  You probably need to increase "current" serial number.')
+
+        # If this preference was added after the saved PrefsVersion we should set defaults
+        if aa >= pv:
+            return True
+
+        return False
+    ###########################################################################
+
+prefsVersion = PrefsVersion()
+###########################################################################
+
 if platform == 'darwin':
     import objc
     from Foundation import NSFileManager
@@ -103,7 +167,7 @@ class PreferencesDialog(tk.Toplevel):
         outframe = nb.Frame(notebook)
         outframe.columnconfigure(0, weight=1)
 
-        if not config.getint('PrefsDidSave'):
+        if prefsVersion.shouldSetDefaults('0.0.0.0', not bool(config.getint('output'))):
             output = config.OUT_SHIP	# default settings
         else:
             output = config.getint('output')
@@ -502,7 +566,7 @@ class PreferencesDialog(tk.Toplevel):
 
 
     def apply(self):
-        config.set('PrefsDidSave', 1)
+        config.set('PrefsVersion', prefsVersion.stringToSerial(appversion))
         config.set('output',
                    (self.out_td.get()   and config.OUT_MKT_TD) +
                    (self.out_csv.get()  and config.OUT_MKT_CSV) +