open-document-selector: Properly remove idle
It's not possible to use g_idle_remove_by_data when the idle was added with gdk_threads_add_idle_full. gdk_threads_add_idle_full uses g_idle_add_full internally but it creates a temporary data strucuture. The address of this temporary data structure should be passed to g_idle_remove_by_data. For obvious reasons we cannot do that, so let's use g_source_remove.