Make BB-tracks themeable

- This allows defining a default colour for BB-track patterns in the CSS
- The default colour is used for all bb-patterns which don't have a custom colour set by the user: in other words, the colour of a pattern can be any rgb-value OR "style colour"
- By default, all created bb-patterns use the style colour
- You can also reset colourized patterns to use style colour again
- Backwards compatibility: old projects will be loaded so that any pattern using either of the old default colours will be converted to use style colour

TODO: add a settings option that can disable custom colours (ie. always use style colour), and/or an option to reset all patterns in a project to style colour. This is needed, since themes can now change the song editor background, which can lead to unfortunate colour combinations with custom colours...