Support activeFocusOnTab in QQuickItem

Support activeFocusOnTab in QQuickItem

Add activeFocusOnTab as property to QQuickItem. Setting the
property allows automatic keyboard navigation between all
elements that have it set.

This key event handler will only be called after the QML key
handlers, such as KeyNavigation and Keys, and the C++ key
event handlers, such as keyPressEvent function.

Algorithm is most done by Frederik Gladhorn, in cooperation with
Gabriel de Dietrich.

Done-with: Frederik Gladhorn <frederik.gladhorn@digia.com>
Done-with: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Change-Id: I8b58be9c20d113661fe85d27bdb1af84340d9de5
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>