Add a way to conjure side by side comparison from the product page

[teolemon]

Problem

• Add a way to conjure side by side comparison from the product page (hidden by DEV mode ATM)
• Additional products for comparison will be added directly from the side by side compare mode itself

Mockups

Part of

• Create a side by side compare mode (to be hidden away in dev mode for the moment) #4223

[monsieurtanuki]

@g123k recently noted (#4005) that - let me summarize it my way - we didn't use an AppBar for the product page and therefore when we had scrolled down there was no way to know which product it was (except scrolling up).
And at least one suggestion included showing the action icons in the app bar (instead of "somewhere after the scores"):

I think we should fix #4005 first, with the current issue in mind (e.g an additional "comparison" action button).

And we could add a "meta" list called "comparisons" with a direct access from the product list.
Which would be a bit redundant as from every list we can run comparisons, but that would help the user use and understand the comparison tools.

[g123k]

Hi @monsieurtanuki, you may have noticed that I'm a bit quiet these days 😇, and I can't tell you more yet (since it's still a work in progress), but we plan to rethink a few things, including this one.

Don't worry, we keep you in touch