feat(web): prediction casing follows current layer

Fixes #6455.

This involved adding an extra hint to the data passed into the lexical
model layer, being the CasingForm that the current layer wants to use.

Given there are several entry points to the lexical model layer, this
meant a few places where we needed to check the current layer.

I refactored the very long function `InputProcessor.processKeyEvent` to
extract out the `buildAlternates` function. There should be no
functional changes to the `buildAlternates` code. But it makes
`processKeyEvent` significantly easier to read.