Add: pancurses backend

[defiori]

On Unix I don't see a reason not to use termion, but on Windows pancurses adds the PDcurses backend which works quite nicely. (Plus you get Unix ncurses for free, however without Unicode).

[fdehau]

That's a nice addition but to be honest I'm a bit hesitant about adding another backend. Are there any feature that potential users may only be able to use with this new backend ?

[defiori]

I understand your hesitation to keep adding backends, however I think pancurses would be worthwhile. For me two reasons stand out:

• Windows: I initially added the pancurses backend to make tui usable on Windows. I'm aware that crossterm aims to be a full cross-platform terminal library and truly hope they become the go-to solution in Rust. However, their API is still in flux and there are a few significant feature gaps, e.g. keyboard input (see this and this). While I'm aware that tui doesn't aim to deal with input itself, it should still be able to expose reasonable input handlers from the backend.

• Curses: I think it's fair to say that *curses isn't just yet another terminal library but standard for text-based applications (htop, ranger, ..). While that's not an argument in itself, it means that the API is set in stone and there shouldn't be any stability issues down the road. On *nix pancurses is quite a thin wrapper around the system ncurses libraries which some users may prefer to bundling a full terminal library. On Windows it comes with a full terminal implementation (PDcurses win32a flavor) that talks directly to the Windows API, making it fully cross-platform.

In summary, while I know the crossterm devs are working hard on filling in the gaps (e.g. input handling) the pancurses backend already offers full Windows functionality plus a few goodies that will be useful even as crossterm gets more mature.

[fdehau]

Overall this looks good. I've just pushed a change on how the demo examples are organized in order to be able to reuse most of the UI and app state code for all available backends. Could you update the curses_demo to follow this pattern ? With this change, I've also removed the additional backend example so I think you could do the same here.

[defiori]

I just pushed the changes, curses_demo now uses the shared ui / app code and I've removed the standalone curses example. See comment on change request for changes to the backend instantiation.

[fdehau]

LGTM, thanks for taking the time to work on this and for being patient enough with my delayed reviews =)