Add a new method for Box to clear a Box.

box.go

@@ -83,3 +83,11 @@ func (b *Box) Padded() bool {
 func (b *Box) SetPadded(padded bool) {
 	C.uiBoxSetPadded(b.b, frombool(padded))
 }
+
+// Clear clears the children of the Box.
+func (b *Box) Clear() {
+	n := len(b.children)
+	for index := 0; index < n; index++ {
+		b.Delete(0)
+	}
+}