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

[Drew Adams <drew.adams@oracle.com>]

Subject line says most of it.

Face `header-line-highlight' was added in Emacs 26.  It inherits from
`highlight'.  I think it should inherit from `mode-line-highlight'.

I suspect that it was made to inherit from `highlight' to keep the
previous behavior, from before the face was added.

I'd also suggest that `mode-line-highlight' inherit from `highlight' (in
addition to its current default attribute settings).  But you can
consider that to be a separate suggestion.

The first suggestion, to have `header-mode-highlight' inherit from
`mode-line-highlight' changes the default appearance, but it makes sense
by aligning header-line with mode-line, which we otherwise do generally.

And if the second suggestion of having `mode-line-highlight' inherit
from `highlight' is accepted, that too changes the default appearance,
but it makes sense by aligning mouse-face highlighting.

(I don't suggest that all `mouse-face' highlighting in Emacs should be
aligned.  I do think the suggestions here might make sense.)

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''