Generate GDBus proxy object for org.gtk.Application

Having a proxy object will make it easier to monitor the application's
busy state.

This adds a dependency on gdbus-codegen

https://bugzilla.gnome.org/show_bug.cgi?id=736492
Use org.gtk.Application proxy to monitor app's busy state

This simplifies the code and fixes a race where an application could
call g_application_mark_busy() before the shell subscribed to change
notifications on the application's busy state.

https://bugzilla.gnome.org/show_bug.cgi?id=736492
Add busy property to ShellApp

Using a separate property to show when the application is busy rather
than cramming it into the state property makes the code clearer. In most
places we only care if an app is running or not, not whether it is
actually busy.

https://bugzilla.gnome.org/show_bug.cgi?id=736492