Allow hard-coded paths to be overridden with $XDG_DATA_DIRS

Necessary for post-installation relocatability.
Also ensure that data installed to share/gramps is in mac bundle.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21246 4ae1f11a-8b86-4847-b8af-ab372f36d1fd