generate.py: do not use brushsettings.py
generate.py: do not use brushsettings.py

It's just a code generator script, so it can use ordered namedtuple()
representations for the intermediate stages. The output C headers are
identical to the old system.

This allows the fiddlier interface in bushsettings.py to be migrated to
the MyPaint codebase at our leisure.

achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint