Revert "Allow the Keys attached property to override Action shortcuts"

This reverts commit 82c3b8b70de454569a60d809c855f0af59f48400.
WIP: Allow the Keys attached property to override Action shortcuts

The Keys attached property was never getting a chance to handle the
ShortcutOverride event. This change adds a shortcutOverride signal
to QQuickKeysAttached, allowing Qml to handle ShortcutOverride
events.

Task-number: QTBUG-32928

Change-Id: I4280eb234e6a0a4351bad45adeb4d8ff28cfb20f