Simplify channel proposal acceptance

[matthiasgeihs]

The current way of accepting a ledger channel proposal lcp is as follows.

acceptance := lcp.Accept(part, nonce)
r.Accept(h.conn.Context(), acceptance)

1. It is confusing that accept needs to be called on the proposal and the proposal responder.
2. In most cases we provide a nonce via client.WithRandomNonce(), which does not depend on any parameters.

Regarding 2.: We can make the nonce parameter optional and let the default behavior be that the nonce is chosen as client.WithRandomNonce().

Regarding 1.: We should consider combining lcp.Accept and r.Accept into one function. We might even consider combining lcp and r into one object. Alternatively, we should at least rename lcp.Accept to make it clear, that it does not accept the proposal, but only generates the acceptance message.