[TagsFileItemAction] Only show action when file is writable

Summary:
When a file is read-only, tags can neither be added nor removed, and
trying to do so fails silently.

Test Plan: open context menu on readonly file

Reviewers: #baloo, #dolphin, nicolasfella, ngraham

Reviewed By: #baloo, #dolphin, ngraham

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D20982