bz19899: Ensure correct order of list-changed callbacks on GTK.

The code assumes that we set up our new GTK TreeModel in
ItemListModel.on_list_changed() before the
ItemListController.on_list_changed() code runs.  Before it didn't
always run that way.  The new code ensures the correct order of
callbacks.
bz19833: fix pause/resume context menu logic
bz19891: fixed ItemInfo attribute name