Structure handlers to allow .get() to return lists

See #1154. In the end I didn't modify the Attributehandler and
TagHandler like this, instead I added the `return_list` argument
for cases when one wants a guaranteed return.

evennia/commands/cmdsethandler.py

@@ -544,9 +544,9 @@ class CmdSetHandler(object):
     # legacy alias
     delete_default = remove_default
 
-    def all(self):
+    def get(self):
         """
-        Show all cmdsets.
+        Get all cmdsets.
 
         Returns:
             cmdsets (list): All the command sets currently in the handler.
@@ -554,6 +554,9 @@ class CmdSetHandler(object):
         """
         return self.cmdset_stack
 
+    # backwards-compatible alias
+    all = get
+
     def clear(self):
         """
         Removes all Command Sets from the handler except the default one