statusline: implement per-account status

Implement a statusline state for each account. Keep the ex line and the
push notifications global. Add account name prefix to push
notifications. Prefix status line with account name when multiple
accounts are available.

Use account-specific status line for each tab where an account is
defined.

Handle threading, filter/search, viewer passthrough and connection
status.

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

commands/msg/toggle-threads.go

@@ -3,6 +3,7 @@ package msg
 import (
 	"errors"
 
+	"git.sr.ht/~rjarry/aerc/lib/statusline"
 	"git.sr.ht/~rjarry/aerc/widgets"
 )
 
@@ -34,6 +35,7 @@ func (ToggleThreads) Execute(aerc *widgets.Aerc, args []string) error {
 		return err
 	}
 	store.SetBuildThreads(!store.BuildThreads())
+	acct.SetStatus(statusline.Threading(store.BuildThreads()))
 	acct.Messages().Invalidate()
 	return nil
 }