Added ofxUIEnvelope, ofxUIEnvelopeEditor and associated changes in ofxUICanvas.h and ofxUI.h

[mitchmindtree]

Hey Rez!

Here's the src for the envelope editor I was showing you. Hope it's not too messy! I tried commenting in most places, hopefully it all makes sense. Let me know if it doesn't!

If anyone else reading this wants to see it working, here's a short example where i'm using it to control amplitude and frequency envelopes in an instrument editor.

https://www.youtube.com/watch?v=BK0f3CicNR4

Cheers,

Mitch

[mitchmindtree]

Closing - going to edit this into the new .h / .cpp format