color: add a 'ui.color' option to control color behavior This new option control whether or not color will be used. It mirror the behavior of '--color'. I usually avoid adding new option to '[ui]' as the section is already filled with many option. However, I feel like 'color' is central enough to deserves a spot in this '[ui]' section. For now the option is not documented so it is still marked as experimental. Once it get documented and official, we should be able to deprecate the color extensions. There is more cleanup to do before that documentation is written, but we need this option early to made them. Having that option will allow for more cleanup of the initialisation process and proper separation between color configuration.

color: reinvent dictionary move the module lever dictionary declaration to a more standard (and practical indentation)

chg: deduplicate error handling of ui.system() This moves 'onerr' handling from low-level util.system() to higher level, which seems better API separation.