color: move 'write' logic to the core ui class
One more step, the support for writing color is not directly in core. No
behavior change for the default case ('_colormode' = None).
Here are the details of what we have to change to the core method:
* apply to 'self.label' to input in the buffered case
* dispatch to 'win32print' when applicable
* apply to 'self.label' to input when applicable
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.