Make sure we don't squeeze out the ActionButton

Summary:
Use the implicit height to set its height, we're using it to put it on its Loader. This would sometimes resize the Buttons to 0 and we just wouldn't get any buttons at all.

BUG: 406678

Reviewers: mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D20771