Bug 334095

Instead of showing a list of identities from settings and picking one of them at the time a reply action is started, the default identity should be determined only after all recipients are entered. Given that the user has three identities, these options should be offered to the user:

- Automatically determine (current: a@example.org)
- Foo Bar <a@example.org>
- Meh Meh <b@acme.com>
- Test User <xyz@company.com>
- Custom address...

The actual text of the first option should be dynamically updated to reflect what recipients were entered. As an example, starting with no recipients, a@example.org is picked as it's a default identity. When the user enters a first recipient, somebody.else@company.com, the text should become "Automatically determine (current: xyz@company.com)". When foo@acme.com gets added after that, the text should change to "Automatically determine (b@acme.com)" because that domain has a higher preference (it's listed "more to the top" at the list of identities).

Upon picking "Custom address", a new widget should pop up where the user is able to enter any valid e-mail address.

The QComboBox which is used should be changed back to not accepting user-defined options.

Bonus points for handling resets of the underlying sender identity model (as happens when reloading settings).