Expose the client geometry in Toplevel

Even though there is a connection between the frame geometry and the
client geometry, it doesn't mean that frame geometry updates and client
geometry updates are correlated.

For example, if the border size has been changed, the frame geometry
won't change, but the client geometry will.

This change introduces yet another kind of geometry that describes the
rectangle on the screen occupied by the client window. Furthermore,
given that frame and client geometry updates are independent, a new
signal is introduced, which can be used for the purpose of monitoring
client geometry updates.