[wayland] Minimize transients together with parent

Summary:
If a parent window is minimized, then its transients have to be
minimized as well, so the minimize behavior is the same on both
X11 and Wayland.

BUG: 401950
FIXED-IN: 5.15.0

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17570