Very many testcases, some GLib tools (resource compiler, etc) and
GApplication were calling g_type_init().
Remove those uses, as they are no longer required.
https://bugzilla.gnome.org/show_bug.cgi?id=686161
When the application is using its D-Bus backend, it is useful to be able
to export extra D-Bus objects at the right time, i.e. *before* the application
tries to own the bus name. This is accomplished here by adding a hook
in GApplicationClass for this; and a corresponding hook that will be called
on unregistration to undo whatever the register hook did.
Bug #675509.
