Changing the type of reply (private/to list/...) within the composer window

Gui: Change the reply mode from the compose window by means
of a dropdown menu from a tool button.

Switching to another (available) reply mode updates the recepients
list in the open compose window. If the "Hand Picked Recipients"
mode is picked, no action is triggered. Making manual changes to
recipients list reflects in the reply mode dropdown as a switch to
"Hand Picked Recipients" mode.

BUG: 321153
REVIEW: 118802