Add Friend field to Call.
Make the ToxAvCall constructor internal.
Throw exceptions instead of returning error codes in the AV library.
Return Call in ToxAvCall.Call
Forgotten public ToxException change.
Make Tox instantiate ToxAv on demand.

This allows us to move the API in a nice extension.
Rearange arguments and create default codec overload.

This way the code can look even nicer!
Move Cancel to ToxAvCall.
Remove the reason strings in Cancel and Reject.

They are not used anyway and will be removed in the new api.