GTK+ 3.0 introduced gtk_tree_model_iter_previous() so we no longer need to
keep this helper function.
Also, simplify the use of GtkTreeIters by allocating them on the stack.
https://bugzilla.gnome.org/show_bug.cgi?id=652374
Setting updating_settings in update_layouts_list() and checking for it in the
gsettings 'changed' signal handler doesn't prevent a loop because the signal
handler is only called after update_layouts_list() returns and thus
updating_settings is already FALSE at that point.
By using the 'drag-end' signal instead of 'row-inserted', etc. we continue to
support DnD re-ordering of layouts while avoiding the loop from
xkb_layouts_fill_selected_tree() deleting and inserting rows into the model.
https://bugzilla.gnome.org/show_bug.cgi?id=652361
By making sure we only store the layouts in the list store, and keep
GSettings in sync with the list store, rather than constantly re-reading
from GSettings.
https://bugzilla.gnome.org/show_bug.cgi?id=644783
