Add InstantMessaging to .desktop Categories.

Making full set Categories=GNOME;GTK;Network;InstantMessaging;

Some distros mandate full usage of the freedesktop spec.
Alternative to using InstantMessaging here would be Chat

fractal-gtk/res/org.gnome.Fractal.desktop

@@ -7,5 +7,5 @@ Icon=org.gnome.Fractal
 Type=Application
 StartupNotify=true
 X-GNOME-UsesNotifications=true
-Categories=GNOME;GTK;Network;
+Categories=GNOME;GTK;Network;InstantMessaging;
 Keywords=Matrix;matrix.org;