kcms/tabbox: Expose shortcuts as a KConfigSkeleton

Provide a new KConfigSkeleton (ShortcutSettings) so that the
KCModuleData knows if any of the shortcuts is non-default.

This improves the UX by showing an "orange dot" in system
settings when just a shorcut has been changed.

We also need to implement a new specific KConfigSkeletonItem
(`ShortcutItem`) that uses KGlobalAccel to retrieve and store
the shortcuts, instead of storing them in a config file.