Automatically select correct identity when replying

[jjongsma]

When replying to an email, try to select the
appropriate identity based on the address in the "To"
or "Cc" fields of the original email, rather than
always using the default.

I.e., if the email was originally addressed to
"bob@example.com", search for an identity that has
"bob@example.com" as the email address and use it by
default in the reply.