ActionManager: Remove QShortcut registration API

Registering QShortcuts doesn't solve any problem that is not already
solved by registering QActions, and shortcuts are in fact much more
limited (not being able to register multiple shortcuts for different
contexts).

Change-Id: I9478e601b2cbc3c5e12fb5baee43cacc20d0fb9c
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>