[TagsFileItemAction] Hide tags action if XAttrs are not supported

Summary:
Showing the tagging menu when no tags can be added to the file is
pointless, and adding a tag would silently fail.

Test Plan:
create a file on e.g a FAT filesystem
try to add a tag

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

Reviewed By: nicolasfella

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D20858