introduce BaseTheme to allow inheritage between platform themes

Summary:
BaseTheme is the new component where all common definitions should go.
When something specific needs to be done, it should go in the appropriate
platform specific theme

Test Plan: linux version is unchanged but its theme component is now empty

Reviewers: astippich, januz

Reviewed By: astippich

Differential Revision: https://phabricator.kde.org/D15437