Internal windows cannot be minimized

Summary:
This change ensures that the DebugConsole does not have a minimize
button.

BUG: 372000
FIXED-IN: 5.8.4

Reviewers: #kwin, #plasma_on_wayland, broulik

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3244