Set up a controller to detect mouse clicks

hex-grid/src/main.rs

@@ -257,7 +257,13 @@ impl ObjectImpl for HexGridWindowPrivate {
                 }
             });
         }
+        let click_controller = gtk::GestureClick::new();
+        click_controller.connect_released(|_, press, x, y| {
+            println!("key pressed: {} {} {}", press, x, y);
+        });
+
         self.drawing_area.add_controller(&motion_controller);
+        self.drawing_area.add_controller(&click_controller);
     }
 }