Update window geometry when creating the decoration in ShellClient

Summary:
When managing a new decorated ShellClient we observed that the window
gets placed with a wrong geometry and on next damage event expands into
a strutted area.

The main reason for this behavior is that the window geometry did not
get updated when creating the decoration. With this change the geometry
gets updated, so that the placement is performed on the correct geometry.

The change itself does not yet cover all conditions. E.g. a maximized
windows which will get decorations will be incorrectly sized. This
requires further changes.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D1812