Layergroups with : in name will have first portion of name removed in Virtual workspace

This is a fix for http://jira.codehaus.org/browse/GEOS-5821.

For the fix, the workspace prefix object is passed to the NameDequalifyingProxy and instead of checking for the presence of ':'.  The name is checked to see if it starts with the workspace prefix.  If it does not then the full name is returned.

A second change is made so that if for some reason (very unlikely) the layer name is:

    ws:namep1:namep2

then stripping the prefix will return:

    namep1:namep2

instead of:

    namep1

which was the previous behavior.