On Mac only editable ComboBox should receive tab focus

[ChangeLog][QtQuickControls] Mac: ComboBox will only get tab focus when it is
editable.

Change-Id: Ife04de67b2e3dea77ba878b247ad0b676c879c02
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>