Add FileItemActionPlugin for tag manipulation

Summary:
Make it possible to add/remove tags from the context menu. It shows all known tags. Triggering an action adds/removes the appropriate tag. It also allows to create a new
tag.

Test Plan: Add tag to a file. Tag appears in context menu of all files. Add it to another file. Remove it again. Create new tag from context menu.

Reviewers: Baloo, broulik, ngraham, Dolphin, VDG

Reviewed By: Baloo, ngraham, Dolphin, VDG

Tags: Baloo

Differential Revision: https://phabricator.kde.org/D16872