gvle: fix "Modeling plug-in List" window closing

The Modeling plug-in List dialog can not be close if we use the cross in
title bar. We use correctly the Gtkmm::dialog::run and
Gtkmm::dialog::hide functions to fix it (closes #139).
travis: update to use libgtkmm-3.0-dev
README: switch dependency to Gtkmm 3.0