Convert XmppDiscoService into a GObject

This will enable porting things to binding/expression lookup.

I probably didn't _really_ need to make everything properties, but that was easier to do than figure out which ones would really be needed.

Testing Done:
Ran discovery on `pidgin.im`; saw `conference.pidgin.im` show up, then expanded it and the various rooms appeared, as they should. Clicked Add on a few and verified that the Add Chat dialog appeared with the correct values.

Reviewed at https://reviews.imfreedom.org/r/2210/