Cocoa: keep window focus and keep it exposed if going into borderless mode

Change-Id: If39ab50607b3faa09095a5e0418c89931884bffc
Revert earlier revert of qcocoamenu changes since qquickmenu has been patched

Change-Id: I0e0a0c75cba41db7f6cef9c6443337eadbc0b9e5