Per Plugin minSdk Level

Summary:
Each plugin gets a minSdk level assigned. If this level isn't available the plugin doesn't get loaded. This can simplifie the plugin code by removing reccuring Sdk version checks.
Should we let the user know that a plugin is not available due to that or should it fail silently?

Actual Plugin adaptions come in follow-up patches.

Reviewers: KDE Connect, mtijink

Reviewed By: mtijink

Subscribers: mtijink, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D9546