Support modifier mouse/scroll action on internal decorated windows

Summary:
This fixes the problem that alt+lmb did not start unrestricted move
resize for the Debug Console.

BUG: 374880
FIXED-IN: 5.12.3

Test Plan: New test case and manual testing whether alt+lmb/rmb works

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10440