emu/ioport.cpp: Allow controller files to override input sequences for
inputs that don't use defaults, and to override the toggle setting for
digital inputs.
emu/config.cpp: Expose configuration level (mostly matters for
controller files), improved verbose diagnostic messages, and moved a few
things out of the global and preprocessor namespaces.
docs: Added documentation for some controller configuration file
features. The device mapping feature documentation will be merged in at
some point.
util/unicode.cpp, emu/input.cpp: API cleanups.
