lib/model: Refactor override implementation into sendOnlyFolder

I'm trying to slowly clean this up a bit, and moving functionality out
into the folder types and having those methods not reach into model is
part of it. That can mean takign some odd arguments in the meantime,
some of those should probably become interfaces or properties on folder
in the long term.

lib/model/folder.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"github.com/syncthing/syncthing/lib/config"
+	"github.com/syncthing/syncthing/lib/db"
 	"github.com/syncthing/syncthing/lib/ignore"
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/sync"
@@ -167,6 +168,8 @@ func (f *folder) Serve() {
 
 func (f *folder) BringToFront(string) {}
 
+func (f *folder) Override(fs *db.FileSet, updateFn func([]protocol.FileInfo)) {}
+
 func (f *folder) DelayScan(next time.Duration) {
 	f.Delay(next)
 }