support X-KDE-OnlyShowOnQtPlatforms

Summary:
some kded modules can only run in one platform,
especially only xcb or only wayland. this
makes kded support the X-KDE-OnlyShowOnQtPlatforms
entry in the desktop file

Test Plan: touchpad and keyboard kdeds not loaded in wayland

Reviewers: Frameworks, Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: dfaure, plasma-devel, broulik, Frameworks

Tags: Frameworks, Plasma

Differential Revision: https://phabricator.kde.org/D6899