Improve automatic keyboard layout detection

[spoenemann]

Keyboard layout detection works quite well in Chrome since it supports the keyboard-map API. For browsers that don't support it, we currently fall back to guessing based on the navigator.language property, which is very error-prone because many developers (like me) use English as OS / browser language, but a non-US keyboard layout. #4879 suggests adding a UI to manually select a keyboard layout. We should have that UI, but in addition we should try to improve the automatic detection.

A possible approach for this could be to "listen" to the user's keyboard events and evaluate their KeyCodes against the list of known keyboard layouts.