[TagsFileItemAction] Move expensive tag retrieval out of the loop

Summary:
Fetching the tags from the UserMetaData causes two syscalls typically,
repeating it for each known tag is unnecessary work.

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

Reviewed By: #dolphin, elvisangelaccio

Subscribers: meven

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D20856