Menus: Move private items into Controls.Private module

Keeping the same name as before to avoid any confusion when
importing the private module in other places.

Change-Id: I77986cb9ae4de40d83a2d477487dd806a0cb99d8
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Expose PopupWindow type privately

Still needs some API refinement.

Includes manual test for key event handling. It did not work
as expected on Mac, hence this test. Patch pending in integration
in qtdeclarative.

Change-Id: Id88f5f84d2534cf9b9d93f9224207e661894189d
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>