When auto-switching the mode back to the previous mode
after debugging finishes, floating debug windows
were in a funny state, if at the same time Qt Creator
was (re-)activated as the previous application by the
window manager. Work around that by doing the switch
asynchronously.

Task-number: QTCREATORBUG-14689
Change-Id: Iec1246854843f523b29003b1997466926ea02402
Reviewed-by: hjk <hjk@theqtcompany.com>