[r22949]Bug 7017 GUI stuck when external viewer launched

Use a Glib timeout event to poll the open process and handle
any error returns

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps34@22952 4ae1f11a-8b86-4847-b8af-ab372f36d1fd