[TagsFileItemAction] Ensure file tags are included in shown tags

Summary:
A file may be located outside an indexed path and have tags which are
not contained in the baloo index DB. Make sure all file tags are shown
in the context menu.

Reviewers: #baloo, #dolphin, nicolasfella, ngraham, elvisangelaccio

Reviewed By: #baloo, #dolphin, ngraham

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D20857