Add wxListCtrl::IsVisible()

Allow checking if the given item is (at least partially) visible on
screen.

Closes https://github.com/wxWidgets/wxWidgets/pull/1444

Closes #9949.

include/wx/generic/listctrl.h

@@ -130,6 +130,7 @@ public:
     bool EndEditLabel(bool cancel);
 
     wxTextCtrl* GetEditControl() const;
+    bool IsVisible(long item) const wxOVERRIDE;
     void Edit( long item ) { EditLabel(item); }
 
     bool EnsureVisible( long item );