Issue 30

Provide support for multiple identities.  Identities can be managed in
the Account Settings.  While composing a message, an identity can be
chosen for sending.  Identity information and signature edits are
saved in a header field, so that they survive being synced to the
server and back.

Provide support for editing the quoted text, either for replies or
forwards.  The quoted text is immediately editable, in a separate
editor from the main body.  When saved in a draft, the two are
contatenated, but the length of the main body is saved with the
identity information, and is used to split the two parts again, when
opening the draft.