gapplication: add bind_busy_property()

Balancing g_application_{un,}mark_busy() is non-trivial in some cases.

Make it a bit more convenient by allowing to bind multiple boolean
properties (from different objects) to the busy state. As long as these
properties are true, the application is marked as busy.

https://bugzilla.gnome.org/show_bug.cgi?id=744565