The chooser connects to the default account manager automatically.
Additionally, re-parent the `GtkTreeModelFilter` subclasses to `GtkFilter`, and use them on the `filter` property.
This also means the previous `filter_func` that was moved to `GtkFilter` in /r/1995 are now used again.
Testing Done:
Opened most of the affected dialogs. Disabled XMPP account and saw that it was removed from all connected-only filtered choosers. Re-enabled account and it was back in the choosers.
Confirmed that only XMPP, and not Demo accounts appeared in the two XMPP-specific plugins.