Some cleanup

With latest changes WndProc doesn't have any effect anymore.

Here the outline panel still suffers from flickering, but only in some extreme redrawing cases and I think it's not directly caused by the TreeView itself.

The container form could be declared with:

    protected override CreateParams CreateParams { 
      get 
      { 
        CreateParams cp = base.CreateParams; 
        cp.ExStyle |= 0x02000000; // WS_EX_COMPOSITED
        return cp; 
      } 
    } 

But didn't want to touch that part.

Would like to hear if other people notice improvements, because I definitely do in two different machines.