Revert "Decided to remove uiControlVisible(). No idea why I had it there in the first place; if it was for testing then the actual tests do a good enough job of that."

Oh right I needed it for uiBox.

This reverts commit 9de44c1346.

new/ui.idl

@@ -49,6 +49,7 @@ interface Control {
 	func SetParent(c *Container);
 	func PreferredSize(d *Sizing, width *intmax_t, height *intmax_t);
 	func Resize(x intmax_t, y intmax_t, width intmax_t, height intmax_t, d *Sizing);
+	func Visible(void) int;
 	func Show(void);
 	func Hide(void);
 	func Enable(void);