color: move '_colormode' to the core 'ui' class
Having all 'ui' objects aware of 'color' allows us to update the core code to
handle color. The mode will stay 'None' in the default case so that will not
introduce any changes.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.