Fixes#6364.
Removes `&layerChanged` and adds `&newLayer` and `&oldLayer` system
stores with the additional nuances around values as described in #6364.
The Caps Lock change I made in the previous commit broke
desktop Caps Lock handling. The fix was a little more
convoluted. Turns out that _UpdateVKShift is also pretty
crufty. I simplified it, removing dead code.
Selecting the new Caps Lock `caps` layer was not correctly setting the
layer state in the key event to `caps`, which caused Caps Lock rules
to fail to match.
Relates to #5853 and others.
Selection management was not working properly with the various
OutputTargets:
1. When there is a non-empty selection, rules have no context -- it's
like new text.
2. Backspace over a selection deletes just the selection.
3. Typing a character replaces the selection, of course, and collapses
the caret to the end of the new text.
4. `hasSelection` is a very strange name for `OutputTarget` descendants.
It doesn't mean "has an active selection" but rather, kinda means
"supports selection internally".
5. Added `isSelectionEmpty` which is used for some of the new selection
rules above.
Note that the `touchAlias` OutputTarget class does not currently support
selection. I hope we can deprecate `touchAlias` with the use of
`inputMode` (#3030) in the future, rather than adding support for
selection.
Relates to #5853.
Two things happened here:
1. Construction of Mocks made an assumption that the selection should
always be deleted (outputTarget.ts:363). However, for NewContext and
PostKeystroke processes, we don't want to change anything.
2. Even if nothing is changed, the transcription would emit what is
in theory a no-op ruleTransform (insert="", deleteLeft=0,
deleteRight=0). But apps would treat this as deleting the selection.
This fix goes a little broader than I would have preferred, but adds a
readonly mode to the transcription and mock model, so that we can
control explicitly when changes are applied to the text store.
