added "unit_string" for human readable unit values
fixed ModelExtrusion visibility
fixed ModelSupport visibility
added "Models" dependency to OpenGLWindow (temporarily: until the offending calls are gone)


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@1141 bbaffbd6-741e-11dd-a85d-61de82d9cad9