Also 'gtk-icon-sizes' in themes are deprecated and ignored. That
means that panel-menu and panel-menu-bar always is registered with
default sizes - 24x24px and 16x16px. We can safely replace these by
built-in stock icon sizes.

This commit also restores limited option to change icon size. You
can change icon size to one of built-in sizes - 16px, 24px, 32px
and 48px.