Splitted colorschemes, added better gradient support

- Splitted colorschemes into colors definitions file (TODO: remove non-colors
  definitions like weather_condition_hot and base00) and actual colorscheme.
- Removed dictionary containing groups definitions for all groups for all modes,
  now colorscheme is queried for this each time.
- Moved determination of colors from theme to renderer.
- Added gradients definitions (actually, only one) to new colors file.
- Made line_percent with gradient=True use new gradients.