WIP: show close dialog on toplevel exit #2176

Hello,
Firstly thanks for developing foot terminal.

Well, there are some people out they're so united with their window manager's key binding, that they press close keys accidentally almost too often.
This PR shows a close dialog on top level exit request.

I had no clue what I was doing; so it needs refactoring a lot.
The rendering part is almost identical to the "url-mode". The hardest part is to check if it's ok to shutdown the term or not.
Gnome terminal and Ghostty functions are here and here. One uses the shell integration the other uses thetcgetpgrp() function. I used both.

Thetcgetpgrp() function gives the foreground process, if it's the slave process, then we use shell integration to check if cursor is at the prompt. we also need to check if shell have background process or not. which I have no idea how to do that! One option is to check whether slave has at least a child process or not. Good option in theory but not accurate enough (weird shells with child process) and it's OS dependent. The better option is to ask shells to report their jobs statues. Something like shell integration to detect where prompt is.

And of course it doesn't change the default behavior and should be activated with config file.
Thanks

This was discussed previously in #1473.

@craigbarnes wrote in #2176 (comment):

This was discussed previously in #1473.

Hmm.
So two problems: 1)GUI! 2)Guessing game!

1. This PR doesn't use any GUI toolkit. (Yay!)
2. This is still a problem.

But hear me out:
I completely agree the default behavior of foot is the best at the moment. So in the doc it can be stated that this config option is not flawless.

Tackling the issue, however, I think the best we can do is to have a boolean variable and two specific OSC one indicating that it's Ok to exit and the other not. In this way shells or others processes can speaks with foot. This would be other programs' responsibility.
In this case, we should replace this.

Thank you for considering this.

As an experiment, something like this new branch can do the trick.

and these functions should be added to fish for example:

function foot_cmd_start --on-event fish_preexec
 if jobs -q
 echo -en "\e]133;N\e\\"
 else
 echo -en "\e]133;O\e\\"
 end
end
function mark_prompt_start --on-event fish_prompt
if jobs -q
 echo -en "\e]133;N\e\\"
 else
 echo -en "\e]133;O\e\\"
 end
end

In the end other slave programs should report their status too.

Edit:
Also we can add config for this.

Sorry to say that this is a PR I'm not interested in merging. It's large, adding lots of logic and complexity for a feature there hasn't been much interest in. There's also still no way to really determine whether the close-warning should be displayed or not. I'm not sure relying on an escape sequence is a good idea.