Fixes#6364.
Removes `&layerChanged` and adds `&newLayer` and `&oldLayer` system
stores with the additional nuances around values as described in #6364.
Relates to #3621.
Add support for `&layerChanged` system store. This store is set to `1`
before a `begin postKeystroke`, if the keystroke it follows resulted in
a layer change, either programatically through a keyboard rule, or
through a `nextlayer` property of the touched key.