app-menu: Update clip on icon size changes

app-menu: Update clip on icon size changes

To keep the app icon from overlapping the panel's (border-image)
border, a custom property for clipping the app menu icon's bottom
was introduced. But if the clip region is set before the initial
icon is set, the entire actor ends up clipped. Also due to the double
meaning of clutter_actor_get_height() (e.g. preferred height versus
allocated height), the clip region may end up too large and the icon
overlaps the panel's border-image.
Fix both problems by updating the clip region on size changes as
well, rather than on style changes only.

https://bugzilla.gnome.org/show_bug.cgi?id=644122