More flexible linking of inputs/mixers/overlays/outputs

[matthew1000]

Excuse the massive diff.

Changes:

• Inputs can now link to outputs (without the need of a mixer)
• Mixers can now link to other mixers
• Overlays can now be applied to inputs as well as mixers
• IDs start at 1, not 0 (more intuitive)
• Fewer lines of Python (excluding tests) - simpler, woooo!

Known issues:

• The z-order of mixers in other mixers is always 0

• The web interface can get very complicated mixing mixers into mixers

• [x] Tests passig

• [x] New tests written

• [x] Code linted