Fix 97 - Double click canvas window built in close icon crash

I simply prevent double click by disabling the closebutton

* closebutton become a CanvasView member (underscore removed to respect previous convention of naming members)
* closebutton is "disabled" - set_sensitive(false) - on Canvas_View::close_view
Fix underline accelerator waypoint's contextual menu

using Gtk::MenuItem::set_use_underline