Improve the descriptions and tooltip for GraphViz aspect ratio option

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps34@21631 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
Make availability of GraphViz settings depend on output format

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps34@21632 4ae1f11a-8b86-4847-b8af-ab372f36d1fd