Add keys from USB HID Usage Tables v1.12

This conflicts with atreus.c as the higher-valued keys overlap the
special handling of shift-, ctrl-, alt- and gui-.  See future work.
Fix keycode 134
Use macros to define keys

Now there are no more magic numbers, but the number of layers and
functions are temporarily reduced.  They can be expanded again by
moving the modifiers up to the high four bits of int keypress.

Currently LAYER() does not work while fn is pressed as releasing fn
resets the layer to the base layer once fn is released.  This may have
been true in the magic number version as well.
Add softdvorak C layout

There is no longer support for conversion from JSON in the Makefile
Increase layers and functions

More are available per comment
Restore JSON layout support

Improve Makefile to rebuild layout.h when the layout or the generation
script are updated.
Restore dependency from build to layout
Remove layer set debugging code
Document .h layout macros