BIKESHED: completion faces

[Stefan Monnier]

In Emacs-26, completion faces looked like the following:

- the "common" part shown like the default face.
- the "first-difference" shown in bold.
- everything else uses the default face.

The "common" part is the part applied to the characters that are also
found in the current minibuffer.  E.g. when completing

   M-x ne-li ?

the "ne" and "-li" chars in the *Completions* are the "common" part.

With the basic, prefix completion, the "common" part is not very
important and is separated from the rest at the "first-difference", so
the default faces make a fair bit of sense there.

But for other completion styles such as `substring`,
`partial-completion`, and even more so for `flex`, it's not always
immediately obvious how the minibuffer contents relate to the possible
completions displayed in *Completions*.

For this reason, it is common in other completion systems to
highlight the "common" part somehow (e.g. underline, bold, ...).

I think Emacs's defaults should be changed so that the "common" part
(which uses the `completions-common-part` face) is highlighted somehow.

So, what should we go for:
- bold (but then we need to change the `completions-first-difference`
  face to keep it different, e.g. underlined)?
- underlined (tho I don't like underlined text very much)?
- some foreground color?
- some background color?
- something else?


        Stefan