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: {F6291575}

Reviewers: #krita, rempt

Reviewed By: #krita, rempt

Subscribers: rempt

Tags: #krita

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