appMenu: Clip app icon when the button is active

appMenu: Clip app icon when the button is active

We now use a border image on active panel buttons to underline the
button's content. As the property does not affect the content's
allocation, the app icon ends up being drawn on top of the border
image. To prevent this, use a custom property to clip the bottom of
the app icon when the button is active.

https://bugzilla.gnome.org/show_bug.cgi?id=642697