This is a very pervasive change and basically turns the whole top-level of the GTK-UI bottom-up. If this change turns out right, it would likely solve #1048 WARNING: in parts not implemented, breaks UI
...this (finally) allows us to get rid of these elements within GtkLumiera. Our Model will be represented in a quite different way (via UI-Bus).
the only adequate thing you can do with managers is to demote them.
