* Added description string to operator types, should be set along with ot->idname when defining ops.

* User interface uses this as a tooltip when NULL or "" is given.
* Python doc generation includes this description 
* Python defined ops take the description as an argument.

* NULL check to image_ops.c, was crashing on exit when there was an image open.

source/blender/python/epy_doc_gen.py

@@ -265,6 +265,7 @@ def op2epy(target_path):
 		
 		out.write('def %s(%s):\n' % (op, ', '.join(kw_args)))
 		out.write('\t"""\n')
+		out.write('\t%s\n' % rna_struct.description)
 		for desc in kw_arg_attrs:
 			out.write('\t%s\n' % desc)
 		out.write('\t@rtype: None\n')