Separate stages and patterns

[matsn0w]

Is your feature request related to a problem? Please describe.
Some users have asked questions regarding the creation of asynchronous patterns, especially when multiple light sources are operating at the same time a light stage. This is very hard to 'simulate' with a single pattern.

Describe the solution you'd like
Separate the concepts of 'patterns' and 'light stages': a light stage is a collection of one or more patterns that operate asynchronously and simultaneously. A pattern is a loop of extras being turned on or off for a certain amount of time.

Describe alternatives you've considered
Using two light stages at the same time, but that's giving up one.