Add Rect Transfrom exetnssions

[stan-osipov]

public static void SetLeft(this RectTransform rt, float left) { rt.offsetMin = new Vector2(left, rt.offsetMin.y); }

 public static void SetRight(this RectTransform rt, float right)
 {
     rt.offsetMax = new Vector2(-right, rt.offsetMax.y);
 }

 public static void SetTop(this RectTransform rt, float top)
 {
     rt.offsetMax = new Vector2(rt.offsetMax.x, -top);
 }

 public static void SetBottom(this RectTransform rt, float bottom)
 {
     rt.offsetMin = new Vector2(rt.offsetMin.x, bottom);
 }