#3062 ttk::sizegrip doesn't use native bindings

[Talvo]

The bindings on a [ttk::sizegrip] use Tcl procs which resize the window in real time as the mouse is moved. However, on Windows (depending on OS settings) normal behaviour is usually to show a grey bounding box for the window, which resizes on mouse motion, and only resize the window on button release. (Pressing escape while dragging cancels the operation.) It would be nice if this native behaviour could be included in ttk on Windows to get a truly native look and feel.