started refactoring of GfxObj, pulling common methods into GfxObjBase (temporarily); serves for both upstream (GfxNode) and downstream (Entity etc) refactoring in a next step


git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2126 5a81b35b-ba03-0410-adc8-b2c5c5119f08