Update `newterm()` to return a `Window`

[gyscos]

To make newterm useful, we need a way to access the screen object. This can be done through stdscr() (see #43) or by returning a Window from newterm, so that it matches initscr. This PR does the second option.

[insidewhy]

@gyscos Hi, I need this also. Wondered how/if you worked around it?

[gyscos]

I have not worked around it so far.

This PR currently fails on Windows - I forgot if it ever worked (and something changed), or if I just missed it in the first place.

[TimonPost]

I would like to see this one merged. There was anything wrong with the windows code? I will have a look for you.

[TimonPost]

pdcurses function returns a SCREEN while window::new_window expects an WINDOW

   pub fn newterm(arg1: *const ::std::os::raw::c_char,
                   arg2: *mut FILE,
                   arg3: *mut FILE)
                   -> *mut SCREEN;