Fix horizontal scrolling of wxPropertyGrid header

wxPropertyGridHeader associated with wxPropertyGrid has to be notified about every horizontal scroll of the grid.
New position is sent with dedicated wxEVT_PG_HSCROLL event being handled by wxPropertyGridManager which in turn scrolls the header accordingly.

See #18313.

include/wx/propgrid/manager.h

@@ -544,6 +544,7 @@ protected:
     void OnResize( wxSizeEvent& event );
     void OnPropertyGridSelect( wxPropertyGridEvent& event );
     void OnPGColDrag( wxPropertyGridEvent& event );
+    void OnPGScrollH(wxPropertyGridEvent& evt);
 
 
     wxPropertyGrid* m_pPropGrid;