Fixes#9430.
Adds a 'default' property to subkeys, only used by longpress-type
subkeys:
* Added property to schema and readers/writers
* Added property to touch layout validator
* Added editor for property to layout builder
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.
Improves the .keyman-touch-layout clean schema and adds additional
cleanup during load and save in the Touch Layout Editor.
The compiler already does transforms for the relevant fields to the
formats that KeymanWeb is expecting (in particular, width, pad to
string).
This is preparation for adding hint data to the format as well.
