Fix a condition in the source changed notification handler

Closes #24

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Michal Toman <mtoman@redhat.com>
Check if X display can be opened

Gtk raise too general exception (RuntimeError) which should not be
caught and considered as evidence that X is accessible.

Closes rhbz#984315

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Michal Toman <mtoman@redhat.com>