UpdateClientArea from ShellClient::doSetGeometry if the ShellClient has a strut

Summary:
This ensures that resizing a panel updates the client area. On X11 there
is an event when the struts change, but on Wayland the struts are implied
from window type (panel) and the panel behavior, so we need to trigger it
manually.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D1811