#3063 ttk::sizegrip should be hidden for maximised windows

[Talvo]

On MS Windows, the sizegrip should be hidden when the toplevel it's a part of is maximised. This is the default behaviour in all other Windows apps which have a sizegrip. (I can't speak for other OSes/window managers, but it seems more sensible to me in all cases.)

Failing that, it should at least do nothing (and preferably not change the cursor, either) when the window is maximised - it's very weird that, when the window is maximised, you can drag the sizegrip to alter the size the window will have when deiconified.