Padding is updated for some widgets once, when creating the widgets.
But it seems style may not be loaded yet. Therefore the padding has
to be updated on "style-updated" signal to reflect style changes.
https://bugzilla.gnome.org/show_bug.cgi?id=743180
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
