The only major functional change is that the Track now needs to know
whether it's active or not, in order to listen to the keyboard (or not).
There are some bugs exposed/created by this, mainly:
* KeysWidget sometimes shows phantom notes. Those do not actually exist
as far as debugging has revealed and do not play in the synth.
* The keyboard can lock up Piano when rapidly pressing keys. This
appears to be a HashMap bug; I invested significant time in bugfixing
but got nowhere.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *