holding shift while pressing a view button will only select that view and hide all the others

Fltk/viewButton.cpp

@@ -32,6 +32,13 @@ static void view_toggle_cb(Fl_Widget *w, void *data)
   int num = (intptr_t)data;
   viewButton *but = FlGui::instance()->onelab->getViewButton(num);
   if(but){
+    if(Fl::event_state(FL_SHIFT)){
+      for(int i = 0; i < PView::list.size(); i++){
+        if(i != num) opt_view_visible(i, GMSH_SET | GMSH_GUI, 0);
+        else opt_view_visible(i, GMSH_SET | GMSH_GUI, 1);
+      }
+    }
+    else
       opt_view_visible(num, GMSH_SET, but->value());
     drawContext::global()->draw();
   }