Infrastructure for custom, highly specialized effect user interfaces

Submitted by Jeff F.T. `@jeff`

Assigned to Suhas @suhas2go

Someone needs to pick up my failed attempt from 2013 and finish it.
https://github.com/nekohayo/pitivi/commits/custom_effect_config

This is needed for a bunch of features (color corrector, chroma keying, denoisers, audio equalizers, etc.).

See the "Blocks:" field below for the list of bug reports / features depending on this.