Fix some assertions from GTK
Review Request #446 — Created Jan. 24, 2021 and submitted
Fix assertion on startup due to dark mode window.
For whatever reason, this window must be destroyed, not unrefed.
Fix some assertions on shutdown.
The GTK buddy list GObject is finalized before the Purple buddy list, so when Purple deletes nodes, the GTK list model is NULL, and prints out a bunch of assertions.
Run with debug messages and see they're gone.