Display threads in the message list. For now, only supported by the
notmuch backend and on IMAP when the server supports the THREAD
extension.
Setting threading-enable=true is global and will cause the message list
to be empty with maildir:// accounts.
Co-authored-by: Kevin Kuehler <keur@xcf.berkeley.edu>
Co-authored-by: Reto Brunner <reto@labrat.space>
Signed-off-by: Robin Jarry <robin@jarry.cc>
