In cw 3.9, interfaces are deprecated and replaced with adapters,
yielding a lot of bw compat in many places -- most if this patch is
concerned with the interface bw compat

- cwvreg: interface cleanup

- doc/adapters.rst: interface cleanup

- entities/adapters.py, wfobjs.py: interfaces bw compat

- entity.py: interfaces bw compat, also get_value, delete,
  attr_metadata, has_perm, set_related_cache, clear_related_cache,
  clear_related_cache, related_rql

- predicates.py: score_interfaces & implements

- interfaces.py & mixins.py: 100% gone

- view.py: implement_adapter_compat, unwrap_adapter_compat

- calendar.py, editcontroller.py, ibreadcrumbs.py, navigation.py, xmlrss.py:
  interface bw compat

- treeview.py: salvage one function from mixins.py


Related to #2782004.