Add ability to define new "computed" channels from other channels

[petoknm]

Currently, I don't see how I could implement a simple extension that subtracts two channels and adds this subtracted channel to the UI.
Tbh, I assumed this functionality was already provided in the base software, but recently I learned this is not implemented, so I want to implement it myself.
Currently I'm stuck because I can't proceed in any other way than exporting samples to CSV and doing the math outside of Logic which is super cumbersome. This is the most requested idea on ideas.saleae.com so please make it happen or allow other people to make it happen at the very least.