ENH: update LandmarkRegistration to support RegistrationPlugins

Still some work to do on the built-in plugins, but the framework
is now in place for extensions (like SlicerRT/Plastimatch) to provide
registration plugins.

Affine registration is now a plugin and it works like the previous
version of LandmarkRegistration which supported only Affine.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22792 3bd1e089-480b-0410-8dfb-8563597acbee