Don't allow showing/hiding the drawer when there's no handle

Summary:
If there's no handle and the drawer is modal, we can understand that the
drawer isn't meant to be closed. Disable the mouse area that is used for
closing it when that happens.

BUG: 393776

Test Plan: Can't reproduce the issue in the mentioned bug report

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D12699