Make app.get_all_import_handlers() a bit smarter

should fetch all "designated" handlers in addition to those which are
officially registered

rattail/app.py

@@ -222,15 +222,31 @@ class AppHandler(object):
 
         :returns: List of all registered Import/Export Handler classes.
         """
-        handlers = self.load_entry_points('rattail.importing',
+        # first load all "registered" Handler classes
+        Handlers = self.load_entry_points('rattail.importing',
                                           ignore_errors=ignore_errors)
-        handlers = list(handlers.values())
+
+        # organize registered classes by spec
+        specs = {}
+        for Handler in six.itervalues(Handlers):
+            spec = '{}:{}'.format(Handler.__module__, Handler.__name__)
+            specs[spec] = Handler
+
+        # many handlers may not be registered per se, but may be
+        # designated via config.  so try to include those too
+        for Handler in six.itervalues(Handlers):
+            spec = self.get_designated_import_handler_spec(Handler.get_key())
+            if spec and spec not in specs:
+                specs[spec] = load_object(spec)
+
+        # flatten back to simple list
+        Handlers = list(specs.values())
 
         if sort:
-            handlers.sort(key=lambda h: (h.get_generic_host_title(),
+            Handlers.sort(key=lambda h: (h.get_generic_host_title(),
                                          h.get_generic_local_title()))
 
-        return handlers
+        return Handlers
 
     def get_designated_import_handlers(self, with_alternates=False, **kwargs):
         """