Delay starting kglobalaccel5 till it's needed

There are usages of KGlobalAccel in e.g. KActionCollection of xmlgui
framework which do not require a running kglobalaccel5. But due to the
way KGlobalAccel inits, it always started the runtime part.

This change ensures that kglobalaccel5 is only started once it's actually
needed, by e.g. registering a global shortcut. If an application does
not use global shortcuts the API should no longer trigger a start of the
runtime part.

REVIEW: 124902
CHANGELOG: Only start kglobalaccel5 if needed.