Move the shortcut management in the settings object

Summary:
This uses the new KPropertySkeletonItem facility to allow proper
integration with KGlobalAccel while keeping all the nice semantic coming
from KConfigXT.

This required turning the kconfig_compiler code into a base class and
inheriting from it to introduce the property managed via
KPropertySkeletonItem.

Reviewers: #plasma, davidedmundson, bport, crossi

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25212