.desktop: Add a file, and adjust icon filename to suit

1. `io.edcd.EDMarketConnector.desktop` based on the flathub one, but with
  reference to v1.5 of the spec (linked in a file comment).
2. As the Icon is specified using `io.edcd.EDMarketConnector`, not only
  `EDMarketConnector`, renamed the .png file and updated the single code
  reference to it.

EDMarketConnector.py

@@ -509,7 +509,7 @@ class AppWindow(object):
 
             else:
                 self.w.tk.call('wm', 'iconphoto', self.w, '-default',
-                               tk.PhotoImage(file=join(config.respath_path, 'EDMarketConnector.png')))
+                               tk.PhotoImage(file=join(config.respath_path, 'io.edcd.EDMarketConnector.png')))
 
             # TODO: Export to files and merge from them in future ?
             self.theme_icon = tk.PhotoImage(

io.edcd.EDMarketConnector.desktop

@@ -0,0 +1,13 @@
+#!/usr/bin/env xdg-open
+[Desktop Entry]
+Type=Application
+# Ref: <https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-1.5.html>
+Version=1.5
+Name=E:D Market Connector
+Comment=Utilise Elite Dangerous Journal and CAPI data
+Icon=io.edcd.EDMarketConnector
+Categories=Game;
+Keywords=Elite;Dangerous;EDDN;EDSM;EDDB;Inara;Spansh;
+Exec=edmarketconnector
+Terminal=false
+SingleMainWindow=true