Added an auto-pause preference, added Esc to unpause, & dup'd preferences to solver more menu

[adamthedog]

Closes #389

[mdirolf]

Overall this looks great to me! I think my only notes are:

• Build is failing due to eslint checks. I don't mind taking a look at this if you aren't able to
• I know you asked about it but I think in the end it probably does make sense to add a new SolverPreferencesList(or something) component that takes PrefSettingProps and returns a fragment filled with all of the <li>s - will probably just help to avoid future mistakes. Again I don't mind doing that if you don't have time!
• I think I'd change the text from "Pause solving when clicking away from the page" to "Automatically pause solving when page loses focus" - just makes it a tiny bit more clear in cases where somebody has an OS w/ focus follows mouse, for example.

Thanks for the contribution - I really appreciate it and can't wait to see it live :)