add on_after_layout

src/Widget.cpp

@@ -202,6 +202,8 @@ void Widget::handle_relayout_subtree(RelayoutSubtreeEvent &event) {
     for (auto child : m_children) {
         child->dispatch_event(event);
     }
+
+    on_after_layout();
 }
 
 void Widget::handle_mouse_move_event(MouseMoveEvent &event) {

src/Widget.hpp

@@ -119,6 +119,7 @@ protected:
     virtual void on_activation_update(ActivationUpdateEvent &event) {}
     virtual void on_paint() {}
     virtual void on_layout() {}
+    virtual void on_after_layout() {}
 
     void set_did_init(bool did_init) { m_did_init = did_init; }
     Point compute_window_relative();