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.

Failing to remove the idle when the open document selector is disposed could
result in a crash because the idle function (real_populate_liststore) would
access invalid memory.

Also remove populate_scheduled because it's not needed for two reasons:
1. populate_liststore and real_populate_liststore always run in the same thread
2. if populate_liststore is called before real_populate_liststore is run, there
   is no need to schedule two calls two real_populate_liststore.

Close: https://bugs.launchpad.net/bugs/1646762