Core: Mark shortcut conflicts in keyboard settings input box

Task-number: QTCREATORBUG-6

Change-Id: Ife1f97d24733814e7512dd8bc6584a1d72f66fa8
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>