UI Scaling: Size the Scale bar as per our startup value

If we use the last configured value then the width of the Scale bar
changes as per the user's last setting, even though they might not have
yet restarted for all the rest of the UI to resize.

EDMarketConnector.py

@@ -1068,11 +1068,13 @@ Locale LC_TIME: {locale.getlocale(locale.LC_TIME)}'''
     We scale the UI relative to what we find tk-scaling is on startup.
     """
     ui_scale = config.getint('ui_scale')
+    # NB: This *also* catches a literal 0 value to re-set to the default 100
     if not ui_scale:
         ui_scale = 100
         config.set('ui_scale', ui_scale)
     theme.default_ui_scale = root.tk.call('tk', 'scaling')
     logger.debug(f'Default tk scaling = {theme.default_ui_scale}')
+    theme.startup_ui_scale = ui_scale
     root.tk.call('tk', 'scaling', theme.default_ui_scale * float(ui_scale) / 100.0)
     app = AppWindow(root)
 

prefs.py

@@ -354,8 +354,8 @@ class PreferencesDialog(tk.Toplevel):
             themeframe,
             variable=self.ui_scale,
             orient=tk.HORIZONTAL,
-            length=300 * (float(config.getint('ui_scale')) / 100.0 * theme.default_ui_scale),
-            from_=10,
+            length=300 * (float(theme.startup_ui_scale) / 100.0 * theme.default_ui_scale),
+            from_=0,
             to=400,
             tickinterval=50,
             resolution=10,

theme.py

@@ -116,6 +116,7 @@ class _Theme(object):
         self.defaults = {}
         self.current = {}
         self.default_ui_scale = None  # None == not yet known
+        self.startup_ui_scale = None
 
     def register(self, widget):
         # Note widget and children for later application of a theme. Note if the widget has explicit fg or bg attributes.