[TagsFileItemAction] Avoid readding an existing tag via "New tag..."

Summary:
The value from the input dialog may be identical to an already set tag.
Avoid writing duplicate tags.

Test Plan:

1. Add a tag "Foo" to a file
2. Add "Foo" a second time via "New tag..."
3. getfattr -d <file> should show "Foo" only once

Reviewers: Baloo, Dolphin, nicolasfella, ngraham, elvisangelaccio

Reviewed By: nicolasfella

Tags: Baloo

Differential Revision: https://phabricator.kde.org/D20865