Introduce the client geometry in Toplevel

In most cases, we don't need to react to client geometry changes, but in
code that deals with server-side window decorations, we need to react to
client geometry changes. The problem is that frame and client geometry
updates are not correlated even though there is a connection between the
frame geometry and the client geometry.

This change introduces the client geometry in the Toplevel class in order
to allow monitoring client geometry updates from DecoratedClientImpl.