bug#43926: 26.3; Face `header-line-highlight': inherit from `mode-line-highlight', not `highlight'

[Drew Adams]

> You want to have this face display as both raised-button _and_
> highlighted in color?  Which other applications do something like
> that?

Dunno.  Does it matter which other apps do that?

> Where the 3D face is not available, we do inherit from 'highlight'.

The real point of this particular bug is to have
header-line inherit from mode-line.  Whether they
should inherit from `highlight' is another matter
(secondary).

(And `highlight' itself could by default have the
attributes now used for mode-line constructs.)

Logically, header-line and mode-line are similar,
and inheritance of their appearances makes some
sense.  This includes `mouse-face' appearance.

Whether their `mouse-face' highlighting should
inherit from `highlight' is a different question.
But most `mouse-face' highlighting does, in fact,
use `highlight' by default.