report-gtk: use wrapped text for warning labels

Warning texts were designed to fit into window size but their width
depends on used font properties. Therefore we have to make text
wrapped in order to not resize window.

Closes rhbz#895745

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Richard Marko <rmarko@fedoraproject.org>