Provide utilities to gracefully shut down the GTK event loop
Heyho!

The work on ThreadButler (renamed from Appster) is progressing quite nicely and the docs are shaping up to.
I'm currently wanting to provide a global "shutdown" mechanism for all threads.

For the owlkettle integration I would like some utility to kill that event-loop as well in a graceful manner, in my case from within a timeout proc.
This is mostly a nicety and not a hard requirement, just would be nice.

I've found [`g_main_loop_quit`](https://docs.gtk.org/glib/method.MainLoop.quit.html) which we appear to not have wrapped.

The problem I'm seeing here is that g_main_loop_quit requires access to the loop that needs to quit.
Which is... kinda difficult when I am not entirely sure we have that.
At least a glimpse into mainloop.nim does not inspire recognition of what in there I'd be calling g_main_loop-quit with.

Any idea what the way to go here would be?