Setting Reply-To through the GUI

I was thinking about how to design the UI for this. At first, I wanted
to add a new field between the recipients widget and the subject field,
but doing that would require either a new button for enabling this
field, or an always-on field with limited value for most e-mails.

In the end, I went with reusing the recipient addresses. This matches
well with the fact that it's OK to allow several Reply-To values (within
a single header in RFC5322 serialization). I don't know what is the best
behavior when pressing Enter after entering this new Reply-To; right
now, the code simply pops-up a new Reply-To row, similar to how Cc
behaves.

Change-Id: If90e11ba42af9c7533f4fa340eaedb6e5e2af9a5