[desktop] Change ID from .desktop to .qt

required to publish on Flathub, and .desktop is a file extension anyway so it's a bad idea
also some improvements to metadata structure, and add branding colours
and fix the welcome message in qt

Signed-off-by: Harry Bond <me@hbond.xyz>

qt/res/Info.plist.in

@@ -24,7 +24,7 @@
     <string>@BUNDLE_EXECUTABLE@</string>
 
     <key>CFBundleIdentifier</key>
-    <string>app.organicmaps.desktop</string>
+    <string>app.comaps.qt</string>
 
     <key>NSPrincipalClass</key>
     <string>NSApplication</string>