Support for multi-line plot titles and introducing subtitles (also supporting multi-lines)

[PaulWessel]

Description of proposed changes

See #4557 for context. This PR implements two related features:

1. It introduces a new +ssubtitle modifier to the -B frame settings. The +s requires the presence of +t.
2. It introduces @^ (and #break;) as markers that break a title or subtitle into multiple lines.

I have added two examples that demonstrates these (longtitles_hat.sh.sh and longtitles_break.sh).

Given that @^ is pretty unique for breaking the line, do we still need #break;? Let me know if I can remove that before we merge.

If you feel the subtitle could be a tad smaller (it is 20p while the title is 24p) let me know. Below is what it looks with 24p/18p, which I think looks better personally:

Later, I will introduce the same to subplot -Theading option as welo, but currently its implementation requires me to make some changes first.

[joa-quim]

Given that @^ is pretty unique for breaking the line, do we still need #break;?

It's not that I dislike @^ by itself, but I don't like so much creating an escape sequence that it's only to be used in one single case (breaking title strings). So, for me we should only have #break (though I still prefer <break>).

24p/18p Looks better to me.

[PaulWessel]

Happy to entertain opinions on this. I can imagine a near future where @^ could work in any other setting as well (say axes lables), so in that case perhaps @^ is not a bad idea anyway.

[PaulWessel]

Now implemented with 18p size.

[maxrjones]

and/or @^ seem fine to me. I agree with Joaquim that is very intuitive.

[joa-quim]

Ok, so leave @^ and #break (the idea for longoptions is that they talk and @^ doesn't)

[PaulWessel]

OK, will merge for now as is. If there were a consensus that <break> is a better choice than #break; then we can change it before 6.2 release.