feat(developer): rework displayHint to defaultHint

After a demo of the Touch Layout Editor @jahorton suggested using a
"default hint" model instead of the "display hint" from the design spec.
I liked this and have reworked the design accordingly. With the default
hint model, a source for the hint text can be specified, such as the
default longpress key, or a specific flick key. This can then be
overridden on a key level.

The hint input element now shows a placeholder to indicate the default
hint for the key, and if a hint has been customised for a given key, it
is shown in green on the key cap.

Changing the default hint does not impact any customised hints.