threading: add backend capabilities to workers

This patch provides a method to report backend capabilities to the UI.
The intial capabilities included in the report are Sort and Thread.
Having these available to the UI enables the client to better handle
server side threading.

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

worker/imap/checkmail.go

@@ -31,6 +31,7 @@ func (w *IMAPWorker) handleCheckMailMessage(msg *types.CheckMail) {
 					Exists: int(status.Messages),
 					Recent: int(status.Recent),
 					Unseen: int(status.Unseen),
+					Caps:   w.caps,
 				},
 				SkipSort: true,
 			}, nil)