Fix QQuickTextInput not overriding shortcuts (del/home...)

[ChangeLog][QtQuick] QQuickTextInput would not accept
delete/home/backspace/left/right keys when the key was used in a
shortcut.

Task-number: QTBUG-34517

Change-Id: I553af8247191ecdadcb4677e9fc85848270a95d3
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>