6932: Check for broken GTK install on linux only

Fix as proposed by jralls.
Also, move the 'gtk30' translation domain name to gramps consts.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@22717 4ae1f11a-8b86-4847-b8af-ab372f36d1fd