Mark Toplevel as not ready for painting by default

Summary:
Get rid of some duplication as InternalClient, XdgShellClient, Unmanaged,
and Client initialize ready_for_painting to false.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24189