[SwipeListItem] Use a real toolbutton to improve usability

Summary:
Use a real toolbutton instead of a custom icon + mouse area, which
provides hovered and clicked states automatically and simplifies
the code a bit.

BUG: 403641
FIXED-IN: 5.56

Test Plan:
Hovered state: {F6605906, size=full}
Pressed state: {F6605907, size=full}

Reviewers: mart, #kirigami

Reviewed By: mart, #kirigami

Subscribers: apol, plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D18908