Unix: Cannot close the display window once opened

[Matt Giuca]

In the Unix/X backend, it is no longer possible to close the graphics window (as of r70, done to prevent crash if you close it then type a graphics command -- see bug #716240).

We need to handle a close event and manually shut things down gracefully so we can start them up again. Either handle DestroyNotify and create a new one the next time you need it (preferable), or set WM_DELETE_WINDOW protocol, to handle it yourself (requires continuous polling even during command-line input). See src/init/xlib/basicimage for the latter.

If the Xlib backend is ported to SDL (one potential plan), this should be easier to manage.