Replace g_idle_add calls with gdk_threads_add_idle.
Replace g_idle_add calls with gdk_threads_add_idle.

All discussions I can find on these basically state that g_idle_add is
for updating the GUI in a threadsafe manner, where as
gdk_threads_add_idle is for more general things surrounding the event
loop, so I think it's best to use that.

nathan-hoad/roland,nathan-hoad/roland