getSlidBallRenderer methods to allow more customization of UISlider (#16532)

* getSlidBallRenderer methods to allow more customization of UISlider look and feel.

* Added const to declaration of getSlidBallRenderer methods for UISlider.

cocos/ui/UISlider.cpp

@@ -753,6 +753,22 @@ void Slider::copySpecialProperties(Widget *widget)
     }
 }
 
+Sprite* Slider::getSlidBallNormalRenderer() const {
+    return _slidBallNormalRenderer;
+}
+
+Sprite* Slider::getSlidBallPressedRenderer() const {
+    return _slidBallPressedRenderer;
+}
+
+Sprite* Slider::getSlidBallDisabledRenderer() const {
+    return _slidBallDisabledRenderer;
+}
+
+Node* Slider::getSlidBallRenderer() const {
+    return _slidBallRenderer;
+}
+
 ResourceData Slider::getBackFile()
 {
     ResourceData rData;

cocos/ui/UISlider.h

@@ -265,6 +265,11 @@ public:
      */
     float getZoomScale()const;
 
+    Sprite* getSlidBallNormalRenderer() const;
+    Sprite* getSlidBallPressedRenderer() const;
+    Sprite* getSlidBallDisabledRenderer() const;
+    Node* getSlidBallRenderer() const;
+
     ResourceData getBackFile();
     ResourceData getProgressBarFile();
     ResourceData getBallNormalFile();