Experimenting with line drawing

[Yuri D'Elia]

I'm not too happy the way threads are drawn.

The default symbols are a bit confusing, and the "fancy" ones are not
what I would expect (I'd prefer the classical line drawing characters).

Also, I cannot really see how default/parent/orphan play in relation to
each other. Especially the distinction from orphan and default seem to
be completely broken.

The best I could come up was this, inspiread a bit by "tig", if you want
to have a quick try:

(setq mu4e-headers-has-child-prefix '(" ┬●") ; Parent
mu4e-headers-empty-parent-prefix '(" ─●") ; Orphan
mu4e-headers-first-child-prefix '("└─●") ; First child
mu4e-headers-duplicate-prefix '("└≡") ; Duplicate
mu4e-headers-default-prefix '(" ●")) ; Default.

I couldn't really line-up anything properly due to the initial prefix,
so I cheated using longer strings.

See:

https://www.thregr.org/~wavexx/tmp/1455725780.png

Without the complete three drawing though, it's still too vague in order
for me to use :thread-subject. I cannot see where a thread ends.