application: introduce methods to mark the application as busy

This feature is intended for clients that want to signal a desktop shell
their busy state, for instance because a long-running operation is
pending.
The API works in a similar way to g_application_hold and
g_application_release: applications can call g_application_mark_busy()
to increase a counter that will keep the application marked as busy
until the counter reaches zero again.

The busy state is exported read-only on the org.gtk.Application interface
for clients to use.

https://bugzilla.gnome.org/show_bug.cgi?id=672018

docs/reference/gio/gio-sections.txt

@@ -2953,6 +2953,9 @@ g_application_open
 g_application_run
 g_application_set_default
 g_application_get_default
+<SUBSECTION>
+g_application_mark_busy
+g_application_unmark_busy
 <SUBSECTION Standard>
 G_TYPE_APPLICATION
 G_APPLICATION