Removed "label" properties which no longer have any function.
Labels are still used as layer names and as identifiers for sending
behaviors between sides of a split keyboard, so those have been left
alone for now.
Aligns *.h and *.c to underscore naming convention.
These were kept (with warnings) for backwards compatibility with external boards/shields:
- kscan-mock.h
- matrix-transform.h
They should be removed in the future.
PR: #523
