Remove Window.clientPos and Window.clientSize properties

The main motivation behind this change is to reduce the number of
geometry properties, in particular limit the use of clientPos that can't
be expressed as an alias to clientGeometry().topLeft().

The client geometry is not useful to scripts as they lack functions to
go from one type of geometry to another. The scripts use exclusively the
frame geometry.

The client geometry is not that useful in kwin too, similar to scripts,
almost all our window management code works with frame geometry. Client
geometry is useful when requesting the client to resize the window.