Cocoa: Improve basic window handling.

Cocoa: Improve basic window handling.

Refactor NSWindow creation into createNSWindow and
setNSWindow. This is necessary to support QMacNativeWidget
where we re-use an already created window.

Implement popup window handling. Make sure the window
is displayed correctly and closes when it should.

Take control over window activation in order to prevent
infinite loops involving the QtCreator "cmd-k" window.
Activation events are for now not sent to popup-type
windows.

There is now a different set of test failures: add
and remove some QEXPECT_FAILs.

Change-Id: I229761b59f90c9815b968eacc2cbc9c20cc5047e
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>