make sure paths in rcc are never derived from absolute paths

Summary:
the paths for rcc plasmoids are always in the form
:/plasma/plasmoids/pluginname/
when setPath is called with a full path, strip out everything
but the last org.kde.*

Test Plan: doubleclick/dnd on widget explorer works again

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13092