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. If the "Hand Picked Recipients"
mode is picked, no action is triggered. Making manual changes to
recipients list reflectes in the reply mode dropdown as a switch to
"Hand Picked Recipients" mode.

BUG: 321153
REVIEW: 118802