Fixes#6364.
Removes `&layerChanged` and adds `&newLayer` and `&oldLayer` system
stores with the additional nuances around values as described in #6364.
Relates to #5815.
build.sh is a new script that builds any or all of the keyboards in the
common/test/keyboards folder. build.bat is a wrapper for build.sh, just
for Windows CMD.
Assumes that kmcomp.exe has already been built or allows you to specify
your own kmcomp.exe if necessary.
Removed the 'heuristic' test as it is probably not appropriate for
testing. Removed the mobile-specific platform from the touch layout as
it did not include a caps layer (and should have been identical to the
tablet one anyway).
Relates to #3620.
Allows keyboard developers to add a Caps Lock layer (called 'caps'),
which will be accessible through double-tap of Shift key, in an upcoming
PR.
Also adds test keyboards to exercise the functionality when it is
available.
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.
