GLProfile: Adding convenient query for highest programmable core only GL profile

As suggested @ http://forum.jogamp.org/How-to-force-core-profile-tp4028307.html

However, one could always get a dedicated core profile via:
  - GLProfile.get(GLProfile.GLES2)
  - GLProfile.get(GLProfile.GL3)
  - GLProfile.get(GLProfile.GL4)

.. etc