ComboBox: Allow setting currentIndex to -1 to clear selection

Task-number: QTBUG-35794
[ChangeLog][QtQuickControls]ComboBox: Allow setting currentIndex to -1 to clear selection

Change-Id: I034b061484ab095a9ec049fb3b35614b98bfb956
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>