7009: endless exception loop starting w/o DISPLAY

Building testing ground for the bug investigation,
after reading the grampsgui.py code...

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@22908 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
7009: endless exception loop starting w/o DISPLAY

constfunc.has_display() was using check_init(),
which, on Gtk3, was happy to initalize even that the display was N/A.
Amended the test with an attempt to actually get the Gdk default display.

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@22909 4ae1f11a-8b86-4847-b8af-ab372f36d1fd