Proposal for experimental layout boolean

I am involved with a project to use landscapeapp to produce an open-source
landscape for my client. They have some layout and rendering changes that
they would like to see made; some of which are general improvements to
Landscape which I believe you may want to use by default, but some which
may need more refinement before their inclusion would be sensible.

As such, we were thinking that it might be a good idea to have an experimental
layout option which could be set in the settings to turn on/off these layout
and design changes, so that individual landscape owners could try them out
and see how they feel. If changes are considered universally improving of the
UX, then we can remove the optionality and make them default.

If this workflow makes sense to you, then we'd propose adding…

```yaml
# settings.yml
big_picture:
main:
alternative_layout: true
```

…to trigger the alternative layout behaviours.

I'd be happy to follow up to this issue with screenshots showcasing some of the
changes we're prototyping, along with relevant `settings.yml` or `landscape.yml` snippets to illustrate how they work.

If I've missed some built-in mechanism for this that is already present, then
please let me know and I'll be happy to switch our patches out to use that.
Otherwise if there's a general consensus that this approach isn't bad, then
I'll provide the setting and our first UI change as a PR soon.

cc @michaelmoss @awright @danielsilverstone-ct @GeriG966

Start by reviewing the proposed settings.yml entry under big_picture.main and the related landscape.yml snippets, then examine the existing layout and rendering configuration described in the issue. The work is complete when the experimental alternative_layout behavior, its configuration, and the proposed UI changes have an agreed scope and are demonstrated with the promised screenshots or a pull request.

Written by the indexing model from the issue text.