Add a + button and the clear button to Add New Tag

Summary:
Add an + button and the clear button to Add New Tag.
It should be easier to use.

BUG: 396392

Test Plan: The new aspect of the menu is:

Reviewers: Krita, rempt

Reviewed By: Krita, rempt

Subscribers: rempt

Tags: Krita

Differential Revision: https://phabricator.kde.org/D15811