gmain: Remove dispatching source stack

This stack exists only to answer the question of "what is the currently
dispatching source" and is handled in a way that makes it very clear
that we don't need to be using a linked list at all...

Just store the GSource directly.

Independently discovered (and same solution) by Phillip Susi.

https://bugzilla.gnome.org/show_bug.cgi?id=709113