Introducing QPA, QML-only MenuBar, Menu, MenuItem

This removes one dependency from QtWidgets.

QPA is only implemented for Mac so far, and depends on qt/qtbase's
I5f83f260602f55b9409ad69abf670afb59b2d33a change. This means, it will
only work with Qt 5.1 and older. Other QPA implementations will
follow, like Android.

The QML-only implementation has been tested mostly on Linux. On Mac,
we need I4716b4c0b6187d14adb34c46b848ca0a16370b5a to be integrated in
qt/qtbase dev branch if we want to enforce non-QPA menus.

Updated ComboBox, ContextMenu to new QPA menu behavior

Includes preliminary documentation for Action, ExclusiveGroup,
MenuBar, MenuItem, MenuSeparator, and Menu.

Change-Id: I6f38849a03a51386465156f8be3034df0d29e40d
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>