Rename element internals to reflect better if they're handling incoming or outgoing data

[bblaszkow06]

@mat-hek wrote: I'm going to rename this and more functions in element internals to reflect better if they're handling incoming or outgoing data in a separate PR

Originally posted by @mat-hek in https://github.com/membraneframework/membrane_core/pull/313#discussion_r775975638

[darthez]

The decision is to evaluate how much work it could be and depending on that make a final decision should we proceed or not.