Ensure that wxGrid appearance always reflects its enabled state

Override DoEnable() in wxGrid instead of Enable() to ensure that the
grid is shown appropriately for its current state whenever either it or
its parent is disabled.

Note that this also fixes the bug with only the main grid window being
refreshed, but not the row/column headers, which also need to be.

src/generic/grid.cpp

@@ -6817,15 +6817,11 @@ void wxGrid::ForceRefresh()
     EndBatch();
 }
 
-bool wxGrid::Enable(bool enable)
+void wxGrid::DoEnable(bool enable)
 {
-    if ( !wxScrolledWindow::Enable(enable) )
-        return false;
+    wxScrolledWindow::DoEnable(enable);
 
-    // redraw in the new state
-    m_gridWin->Refresh();
-
-    return true;
+    Refresh(false /* don't erase background */);
 }
 
 //