Move the list of behaviours used by elements to `Membrane.Element`

[bblaszkow06]

The callbacks for pipelines are in Membrane.Pipeline The callbacks for bins are in Membrane.Bin Why the callbacks for element are in Membrane.Element.Base? Membrane.Element is almost empty, contains only type definitions and one function cc @mat-hek

[mat-hek]

I believe it's to indicate that one should use Membrane.{Source, Filter, Sink}, not Membrane.Element as they do in case of pipelines and bins

[bblaszkow06]

Wouldn't it be enough to mention that in moduledoc? At least this paragraph could be moved to Membrane.Element https://hexdocs.pm/membrane_core/Membrane.Element.Base.html#module-behaviours

[mat-hek]

I think it's a good approach not to have @behaviour or __using__ in Membrane.Element. I agree that most of Membrane.Element.Base moduledoc should be moved there though.

[djanda97]

Would the following be appropriate to move to the @moduledoc section of Membrane.Element?

https://github.com/membraneframework/membrane_core/blob/115eceb34d9b7165748156d511a3524c41b942d8/lib/membrane/element/base.ex#L8-L46

[mat-hek]

Yeah, it seems like a good idea, but in Membrane.Element.Base we'd still need to mention that it's a common behaviour that shouldn't be used directly and one should use Membrane.Source, Membrane.Filter, Membrane.Endpoint or Membrane.Sink instead