Make Clear() virtual in wxItemContainer base class.

This ensures that overridden wxComboBox::Clear() is called even when it's
invoked via a pointer or a reference to wxItemContainer (and not just one
to wxComboBox itself or its other base wxTextEntry).

Closes #16654.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78082 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/ctrlsub.h

@@ -286,7 +286,7 @@ public:
     // deleting items
     // --------------
 
-    void Clear();
+    virtual void Clear();
     void Delete(unsigned int pos);