Rename internal mentions of from_socket and to_socket to sender_socket and receiver_socket

[silvanocerza]

The internal data of the Pipeline graph stores some informations about its edges, like the name of the inputs and outputs of the connected Components.

In newer parts of the code like in InputSocket and OutputSocket we use the terms sender and receiver instead of from and to. Both when talking about sockets and Components.

Older parts use the from and to.

We should change the old cases to be consistent with the new sender and receiver convention.

This is an internal change only and doesn't affect the end user, though it's best to keep things consistent.