QAbstractItemView::selectionCommand() returned the wrong SelectionFlags
when no event is given since c4366ff018.
Therefore re-add the call to QGuiApplication::keyboardModifiers() when
no event is given and add a unittest for them so it's not removed again.
Fixes: QTBUG-89711
Change-Id: I107357df08c4ff1b1a14d49523401c5e7b428f56
Pick-to: 6.0
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>