GrampsLocale: Fix 'Foo has no attribute _Foo__get_type' execption

From moving the set_locale function to the superclass. One can get
carried away with enforcing private attributes...

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@22062 4ae1f11a-8b86-4847-b8af-ab372f36d1fd