composer: Better UI for Cc/Bcc/Reply-To fields
Implemented according to the mockups at https://gitlab.gnome.org/Teams/Design/app-mockups/raw/master/mail/composer.png.
- Use an animated revealer rather than instantly showing/hiding the fields
- Include Cc in the "extended fields"
- Use a toggle button instead of a menu item to show/hide these fields, for better discoverability