- use set_option to set an option, instead of operator[] (no-const)
- keeps a list of OptionManagerWatcher to notify when an option change
it also notifies when an option changes in his parent and the option
is not overridden.
OptionManager map names to options, and may delegate option resolution
to it's parent if it does not contains the asked for option. That way
Buffers can override global options, and Windows can override Buffer
options.
