implement resize feature

src/main/java/ovh/gasser/newshapes/Selection.java

@@ -38,6 +38,10 @@ public class Selection implements Streamable<Shape> {
         notifyListeners();
     }
 
+    public boolean isEmpty() {
+        return selectedShapes.isEmpty();
+    }
+
     public List<Shape> getSelectedShapes() {
         return List.copyOf(selectedShapes);
     }