DBusLoop: remove second layer of watch callbacks where possible

DBusLoop: remove second layer of watch callbacks where possible

Similar to the previous commit, almost every use of DBusWatch can just
have the main loop call dbus_watch_handle.

The one exception is the bus activation code; it's had a comment
explaining why it's wrong since 2003. We should fix that one day, but for
now, just migrate it to a new _dbus_loop_add_watch_full which preserves
the second-layer callback.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=33342
Reviewed-by: Thiago Macieira <thiago@kde.org>