Desktop: Add "SingleMainWindow" key
Signal to the desktop that gajim does not support having an additional main window opended.
https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s06.html
Signal to the desktop that gajim does not support having an additional main window opended.
https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s06.html