Refactor osx_mf_objc_classnames()

extract class_is_cocoa_class_p() from osx_mf_objc_classnames().

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2620 f4f36d40-9719-0410-9868-dde021a23eda