Remove KGlobalAccel bits from Platform

With the addition of KGlobalAccel::globalShortcutActiveChanged(),
there's a way to monitor all triggered actions without adding any
boilerplate code.

This change removes the KGlobalAccel bits from the Platform class. This
reduces the amount of boilerplate code; currently we need to be careful
about ensuring that Platform::setupActionForKGlobalAccel() is called.
Another reason behind this change is to simplify the api of Platform and
remove things that don't have anything to do with outputs.