[applets/appmenu] Port imperative connections back to Connections object

Not sure what exactly didn't work back than, not currently on Qt 5.15
everything seems fine.

Binding.RestoreNone is not the most recommended[1] value, but it
replicates the behavior of one-time assignment which was happening in
Component.onCompleted handler.

[1]: https://www.mail-archive.com/interest@qt-project.org/msg36486.html