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?

membrane_core/lib/membrane/element/base.ex

Lines 8 to 46 in 115eceb

	# Elements
	Elements are units that produce, process or consume data. They can be linked
	with `Membrane.Pipeline`, and thus form a pipeline able to perform complex data
	processing. Each element defines a set of pads, through which it can be linked
	with other elements. During playback, pads can either send (output pads) or
	receive (input pads) data. For more information on pads, see
	`Membrane.Pad`.
	To implement an element, one of base modules (`Membrane.Source`,
	`Membrane.Filter`, `Membrane.Endpoint` or `Membrane.Sink`)
	has to be `use`d, depending on the element type:
	- source, producing buffers (contain only output pads),
	- filter, processing buffers (contain both input and output pads),
	- endpoint, producing and consuming buffers (contain both input and output pads),
	- sink, consuming buffers (contain only input pads).
	For more information on each element type, check documentation for appropriate
	base module.
	## Behaviours
	Element-specific behaviours are specified in modules:
	- `Membrane.Element.Base` - this module, behaviour common to all
	elements,
	- `Membrane.Element.WithOutputPads` - behaviour common to sources,
	filters and endpoints
	- `Membrane.Element.WithInputPads` - behaviour common to sinks,
	filters and endpoints
	- Base modules (`Membrane.Source`, `Membrane.Filter`, `Membrane.Endpoint`,
	`Membrane.Sink`) - behaviours specific to each element type.
	## Callbacks
	Modules listed above provide specifications of callbacks that define elements
	lifecycle. All of these callbacks have names with the `handle_` prefix.
	They are used to define reaction to certain events that happen during runtime,
	and indicate what actions framework should undertake as a result, besides
	executing element-specific code.
	For actions that can be returned by each callback, see `Membrane.Element.Action`
	module.

[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