Implement Assign section

[schulz-alexander]

see also #89

[DrEVILish]

The previous title wasn't that bad. 😆

Feature Suggestion:
On Full size X32 use the Mute Buttons as extra "fixed" assignment buttons.

[schulz-alexander]

The goal is to allow to assign anything to every button/fader/encoder on the X32. The backend is already implemented and I used this to implement the mute groups.

stay tuned 😎

[schulz-alexander]

First iteration of the new configuration page for user defined controls