Include API to generically implement --replace arguments

Summary:
It's quite wide-spread in KDE applications, and having to implement it in every
application isn't very efficient.
This adds an option value to suggest this replace so applications can share the
implementation.

Test Plan: Ported plasmashell to it, works.

Reviewers: Frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, broulik, kossebau, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22946