Center date editor vertically in the grid

Do it for consistency with the other editors and because it looks better
when the date picker is taller than the line height.

src/generic/grideditors.cpp

@@ -1800,7 +1800,7 @@ void wxGridCellDateEditor::SetSize(const wxRect& r)
         rect.SetWidth(wxMin(r.GetWidth(), 2*bestSize.GetWidth()));
     }
 
-    wxGridCellEditor::SetSize(rect);
+    wxGridCellEditor::SetSize(rect.CenterIn(r, wxVERTICAL));
 }
 
 void wxGridCellDateEditor::BeginEdit(int row, int col, wxGrid* grid)