Expose window geometry to PlasmaWindow

Summary: Needed for e.g. the pager.

Test Plan: Test case for PlasmaWindow verifies the geometry

Reviewers: KWin, Plasma on Wayland, hein

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2200