Schedule a workspace repaint in AbstractClient::minimize()

Summary:
We need to schedule a workspace repaint in minimize() to ensure that
there are no ghost windows on Wayland.

Test Plan:
Make sure that window minimization effects are disabled,
minimize a window. Without this patch, you are going to see a ghost
window. With this patch, no ghost window.

Reviewers: #kwin, davidedmundson, apol

Reviewed By: #kwin, davidedmundson, apol

Subscribers: apol, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27516