Stop dbus mainloop before destroying tray on exit

[vynride]

Fixes #2327

The dbus/MPRIS thread runs a GLib.MainLoop that services the SDL tray's libdbusmenu events. It was a daemon thread that was never stopped, so on exit SDL_Quit freed the tray objects while that thread could still be dispatching callbacks on them, causing an intermittent segfault.

This change:

• keeps the GLib.MainLoop as an attribute on Gnome so it can be stopped
• keeps a reference to the dbus thread on Tauon
• on exit, quits the loop via GLib.idle_add, joins the thread with a 2s timeout (logging a warning if it does not stop), destroys the tray explicitly, and only then calls SDL_Quit

[C0rn3j]

Next time, please mark what is your own words and what is AI, same goes for the code.

I'll post up an explanation on your issue in a bit as to why I closed this.

EDIT: Removed the harsh wording

[vynride]

Hey @C0rn3j, sorry about that. I just wanted to solve the error since I kept getting it randomly when I closed Tauon.

I'll keep your review in mind, sorry again.