Ediff review

This issue aims at summarizing our email exchanges about Ediff faces. While so far we focused on the "even" and "odd" hunks, this issue can also be used to discuss Ediff faces more broadly.

To go over my preferences with respect to the even/odd hunks:

I like them to be somewhat noticeable from the corner of my eye, as a navigational aid: they help me keep track of the hunk n will take me to, or the hunk I just left (since Emacs's instantaneous jumps can feel disorienting).
They are not the center of attention though (the hunk at point is), so I feel like they shouldn't be highlighted too much (unlike e.g. Emacs's default highlighting, which makes them pop out almost as much as the hunk at point).
The foreground/background contrast should remain close to the context lines (i.e. the default face); having the even/odd hunks under-emphasized makes them harder to read when perusing the area surrounding the hunk at point.

As far as constraints go, IIRC:

We want to avoid re-using a background that might be used by a major mode (e.g. bg-dim is used by org-block).
I personally don't feel too attached to alternating backgrounds; unless someone else does, we might not need to keep even faces different from odd faces.

Finally, here are the changes to even/odd faces Protesilaos suggested, which I support:

changing the background to bg-alt (closer to the default background than bg-inactive: helps with preference 2),
retaining the major mode's foreground (more noticeable than fg-inactive: helps with preference 3).

Thank you for your work on this!