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/listbase.h

@@ -400,6 +400,9 @@ public:
     bool InReportView() const { return HasFlag(wxLC_REPORT); }
     bool IsVirtual() const { return HasFlag(wxLC_VIRTUAL); }
 
+    // Check if the item is visible
+    virtual bool IsVisible(long WXUNUSED(item)) const { return false; }
+
     // Enable or disable beep when incremental match doesn't find any item.
     // Only implemented in the generic version currently.
     virtual void EnableBellOnNoMatch(bool WXUNUSED(on) = true) { }