add 'onfocus' ui event & optimize container:add

src/graphics/ui/elements/UINode.cpp

@@ -74,6 +74,11 @@ UINode* UINode::listenDoubleClick(const onaction& action) {
     return this;
 }
 
+UINode* UINode::listenFocus(const onaction& action) {
+    focusCallbacks.listen(action);
+    return this;
+}
+
 UINode* UINode::listenDefocus(const onaction& action) {
     defocusCallbacks.listen(action);
     return this;
@@ -101,6 +106,11 @@ bool UINode::isPressed() const {
     return pressed;
 }
 
+void UINode::onFocus() {
+    focused = true;
+    focusCallbacks.notify(gui);
+}
+
 void UINode::defocus() {
     focused = false;
     defocusCallbacks.notify(gui);