[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