So that in the future panels with subpages don't need to have their own
navigation views.
As of now this is useless as implementations of CcPanel will reset the
CcPanel.child object.
Make CcPanel override GtkBuildable, and special-case two types
of children: "content" for the main content, and "titlebar" for
the titlebar. Those two child types exist merely for convenience,
since it's still possible to override the entire panel with
adw_bin_set_child().
For now, no panel is using any of these conveniences.
