remove SENDER_NAME and getExtraSignatureLength() functionality

No gateway was using this anyway. It's easier to add the signature to
the message text manually rather then rely on a gateway to do the same,
and it complicates the things for us.
gateways: remove getMaxParts() function

This is now computed dynamically and for all gateways using the same
approach. No need to complicate the gateway definition.
show the user signature inside the message pane

It's much clearer this way whether a user signature is used or not. Also
it reduces the code complexity a lot.

When sending to multiple recipients, one of the contacts' gateways is picked
and used for signature and coloring (there's nothing to be improved easily
there).