Remove GtkPlacesOpenFlags from public headers

Remove GtkPlacesOpenFlags from public headers

The GtkPlacesOpenFlags enum is only used in private
API, so move it to private headers. Since we still need
a GType for it, add gtkplacessidebarprivate.h to the
headers we use for generating private enum types. In turn,
this registers the other private enums in that header, so
take the opportunity to fix their naming, and use the
generated types for the corresponding sidebarrow properties.

Fixes: #3337