Fix MenuBar related runtime warnings in ApplicationWindow

TypeError: Cannot read property '__contentItem' of null

Change-Id: I0eaf8b9779357e4314ed94335e017e32377195de
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
QML Base style: support transient scrollbars

Change-Id: I4bcbf80f382ad8e972c302a6144c5b42a28f936d
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>