store: extract marking behavior and add tests

Separate the marking functions from the message store and extract the
marking behavior into its own class with tests.

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

commands/msg/pipe.go

@@ -218,7 +218,7 @@ func (Pipe) Execute(aerc *widgets.Aerc, args []string) error {
 			}
 		})
 	}
-	provider.Store().ClearVisualMark()
+	provider.Store().Marker().ClearVisualMark()
 	return nil
 }