Include tcl/tk for tkinter support

[embray]

I've seen one or two people request this before. I don't think it's high priority but it would be worth seeing how much size it adds to the installation. It might be too complicated if it requires an X server and all that which I fear it might...

[slel]

The "topology of 3-manifolds" or "T3M" group provide scripts to
fix Sage on macOS by endowing Sage's Python with tkinter and ssl.

• https://github.com/3-manifolds/fix_mac_sage/

Not claiming this helps do similar things on the Cygwin side...

But if systematically packaging tk makes the installation too large,
the approach of offering a separate script to add tcl/tk and tkinter
for those who want might be a path for Sage-Windows too.

[embray]

The main difficulty with this is not so much with including tcl/tk which is straightforward and not too onerous.

The problem is that the current tk package for Cygwin is configured to be fully reliant on POSIX APIs, but specifically it uses X11 for rendering, so you have to include and run a full X server for it to work. I've tried it before and it does work, but including an X server is a whole can of worms I don't want to get into.

What I wish is that there were some kind of of "hybrid" build of tk for Cygwin, i.e. that uses POSIX APIs where necessary (e.g. to work with Cygwin's filesystem and process management), but uses native Windows APIs for GUI rendering. It seems I'm not the only one who's given thought to such a hybrid approach: https://wiki.tcl-lang.org/page/Building+Tcl+and+Tk+with+Cygwin But it's not trivial and will likely take a lot of effort.

Using an X server might still not be too bad if I can figure out an unobtrusive way to do it without adding too much overhead, but I'm not sure.