Add datehandler unit test

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@22885 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
6871: workaround gtk RTL bug in MultiTreeView

Reapply from gramps40 - blend of Nick's and my fix for the bug.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@22888 4ae1f11a-8b86-4847-b8af-ab372f36d1fd