src/ui.js: Demo version of MutationObserver for activating links.
src/{ui,constants,components}.js, index.css: Figured out new popup system.
- Popups are linked to by actual links to #fragment urls.
- Data for all popups, including glossary, are stored in POPUPS.
  - A single, generic function can construct them all.
- Only implemented for #about popup at the moment.
src/components.js: Popup close button just removes #fragment from url.
src/{ui,constants}.js: Finish implementing new popup system.
src/game.js: Allow directly linking to a popup window via the #fragment.
src/{ui,game,components}.js, index.html: Close popup by going back in history.