combo-box-menu-item: Propagate style changes to the combo menu

combo-box-menu-item: Propagate style changes to the combo menu

ComboBoxMenuItems use ClutterClones to reconstruct the active item
in the associated ComboMenu to not impose a particular MenuItem type
in the menu. However, this results in style changes (for instance
those triggered by icon-theme or text-scaling-factor changes) of
the ComboBoxMenuItem not having a visual effect until the ComboBoxMenu
is shown.
As a fix, force a style update on the ComboBoxMenu when the item's
style changes.

https://bugzilla.gnome.org/show_bug.cgi?id=662799