cc-editable-entry: use GtkStack instead of GtkNotebook

The mechanism of setting label padding had to be changed consequently.

It fixes also following warnings:
(gnome-control-center:32638): Gtk-WARNING **: GtkEntry 0x28bcef0 is mapped but not child_visible
(gnome-control-center:32638): Gtk-WARNING **: GtkEntry 0x28bcef0 is mapped but visible=1 child_visible=0 parent GtkNotebook 0x2b86600 mapped=1

https://bugzilla.gnome.org/show_bug.cgi?id=694325
um-editable-combo: use GtkStack instead of GtkNotebook

The mechanism of setting label padding had to be changed consequently.

It fixes following warnings:
(gnome-control-center:32638): Gtk-WARNING **: GtkComboBox 0x28bcef0 is mapped but not child_visible
(gnome-control-center:32638): Gtk-WARNING **: GtkComboBoy 0x28bcef0 is mapped but visible=1 child_visible=0 parent GtkNotebook 0x2b86600 mapped=1

The ugly hack to catch when combo is losing focus had to be also removed
and notify::popup-shown is used instead. It fixes when GtkComboBox is wrongly
shown instead of GtkButton due to escape is pressed or active item is selected.

https://bugzilla.gnome.org/show_bug.cgi?id=694325