src/ui.js: Allow overlapping popups.
src/{ui,constants}.js: Add about popup code, text, move define() code.
src/{ui,components}.js: Allow images in popups, add logo to About.
src/{intro,components}.js: Refactor 'Video' code.
- Move to 'Media' entity, make 'Video' and 'Image' subclasses of it.
- Remove addBorder, replace with .css('border', [etc]).
src/{ui,components}.js: 'Popup': Replace addImage and addVideo with addMedia.
src/components.js: 'Popup'.addMedia(): Dynamically determine media size.
src/components.js: Adjust 'Popup' coordinate calculation, make colors constants.
src/ui.js: Make About popup reachable by clicking title, move logo position.
src/constants.js: Add depth definition, survey url, more videos.