This patch introduces a mandatory layer attribute to domains. The layer
ordering is superimposed on the stacking order of the views. The
top-most layer can be assigned to a pointer-managing client. An example
for such a pointer is located at os/src/app/pointer. It replaces the
formerly built-in nitpicker mouse cursor.
The new layering mechanism replaces the former "stay-top" session
argument. So the Nitpicker::Connection no longer takes the stay-top flag
as the first argument.
