[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/main.cpp

@@ -133,7 +133,7 @@ int main(int argc, char * argv[])
   UNUSED_VALUE(mainGuard);
 
   QApplication app(argc, argv);
-  app.setDesktopFileName("app.comaps.desktop");
+  app.setDesktopFileName("app.comaps.qt");
   platform.SetupMeasurementSystem();