Allow controlling multiple elements through the same controller

[pahund]

With issue #58, we want to add a check for track definitions that mistakenly assign the same controller to multiple elements. In most cases, this is indeed wrong and should through an error.

There are some use cases where this may be desirable.

Example

We have a channel that plays a Dr. OctoRex with different patterns. Rotary 1 is assigned to the pattern selection of the Dr. When the rotary is dialled all the way down, no pattern is playing, otherwise, patterns 1-8 are selected.

We assign the rotary to both the playing setting of the channel and to the type.