OSX: Get rid of File > Exit menu item

Seems to be a behavior change between Qt 5.2 and Qt 5.3. Now we need to
set the menu role explicitly, text heuristic fails.

Change-Id: I54ecfea5910db4863699bb077b8f8a6c88b56237
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>