ComboBox: Minimize currentText changes when changing model

Bonus change, changing the model after its initial value will reset
currentIndex to 0.

test_arraymodelwithtextrole needs some rework. The issue relies on
textRole and model being set separately, and a late change to
__modelIsArray updating the currentText when it probably shouldn't.

[ChangeLog][QtQuick Controls][ComboBox]
Changing the model after initialization will reset currentIndex to 0

Change-Id: If6c0dee9e022036ef888f2aef87a5c405868684b
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>