Changes for emacs 28

[Ergus]

Hi Emacs:

As 27 has just been released I think it is time to start considering
changes for the release 28. I will add some points I think we could
discuss (specially implying defaults) in order to hear opinions and have
enough time to discuss/implement them.

PLEASE: this are just proposals and every time we talk about defaults we
never get a conclusion or agreement. So I would appreciate if at the end,
one of the main developers (Eli, Stefan, etc) give a final opinion in
favor/opposing/reforming the change and that will close the discussion
(please).

Of course anyone can add extra points these are just the ones I remember
so far.

This are mainly "visible" changes that will benefit new users and first
impression because I don't understand why we hide the best
functionalities until the user learns how to configure them (and some
lisp).

1) Improve the default theme: Maybe set a default dark theme to make
emacs feel more modern. Experienced users usually already have a custom
theme, so they won't be affected. There is actually some statistics
about downloades themes here:

https://emacsthemes.com/popular/index.html

So we can use that to choose or design the theme.

2) Improve the completion buffer behavior: Actually I have a proposal
feature branch for this to make the experience more like zsh; but of
course that is just one option. Most experienced users use IDO, ivy or
helm, icompletes so they won't be affected.

3) Substitute list-buffers with ibuffer: This is actually in the TODO
but for interactive users ibuffer offers a bit more modern
experience. So the proposition is just to set ibuffer to the default
bindings as so far it already offers all the list-buffers'
functionalities.

4) Enable display-line-numbers-mode by default. Almost all editors
around have line numbers by default why we hide them?

5) Enable winmove and mouse-wheel-mode by default.

I think all these are very conservative changes (specially considering
that we have 2-3 years before enabling them)

WDYT??