Reduce gnome dependency of pages and mark pages for specific DEs

[Feichtmeier]

If this app should go any further in terms of "Multi-Desktop" app, we need to mark pages as desktop specific and only insert them if it works on that desktop environment
The best would be if everything would be dbus based ofc and we use the correct keys for those distros. For example mate has different dbus interfaces often etc.
But I think this would be too complicated. So I would prefer the marking approach.
As long as Ubuntu stays gnome based, I do not see a reason to move away from gsettings.dart

CC @wimpysworld