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.

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.
Request API is still only compile-tested.