GsApp: Notify property changes in an idle callback

This makes sure the notify:: callbacks in the UI code get invoked in the
main thread. Without doing so, we can end up calling gtk+ functions from
the plugin thread in response to GsApp property changes.

https://bugzilla.redhat.com/show_bug.cgi?id=1082833

https://bugzilla.gnome.org/show_bug.cgi?id=727823
plugin loader: Remove gs_app_set_state_in_idle

This is no longer needed now that GsApp marshalls all notify:: signals
to the main thread itself.

https://bugzilla.gnome.org/show_bug.cgi?id=727823
plugin loader: Improve thread safety

Add missing locking to two places.

https://bugzilla.gnome.org/show_bug.cgi?id=727823