Fix language selector issues & improvements:
- Editor syntax colors didn't update if you'd ever switched away from that
editor before (DocumentManager was removing too many listeners)
- Show checkmark next to file's current language
- Show "(default)" next to file's default language
- Don't list binary languages in dropdown (restore f8a2b968)
- Properly HTML-escape strings in the dropdown
- Fix JSLint indent errors
- Fix out of date comments
- Fix out of date `isTextField` var name