Support for background color in boxes

[manast]

I have seen that there are old discussions about this functionality.

The reason for creating this new issue is that I believe that this feature is really a must-have, especially when there are no workarounds that I know of (let me know if there are please).

So why is it a must-have? well, because it is crucial for making good UIs in terminals. One of the most important use cases is for creating selections, but also to divide the screen into different areas, implement top and bottom bars, etc.

I post some examples of known terminal apps that use these techniques with good effect:

[vadimdemedes]

Duplicate of #469.