Change reply mode from within the compose window

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

Gui: Put the reply mode menu (QComboBox), the threaded/new thread
toggle option (QToolButton), and the QDialogButtonBox inside
a QHBoxLayout.

Switching to another (available) reply mode updates the recepients
list in the open compose window.

BUG: 321153
REVIEW: 118802