Do not autocreate .props files for multifilesystem backend

This is required as we do not want .props files to be created for nodes,
otherwise they'll stop being considered as nodes, which will break
discovery of calendars.

This fixes Kozea#208

radicale/storage/multifilesystem.py

@@ -23,10 +23,12 @@
 """
 
 import os
+import json
 import shutil
 import time
 import sys
 
+from contextlib import contextmanager
 from . import filesystem
 from .. import ical
 
@@ -96,3 +98,20 @@ def last_modified(self):
             os.path.getmtime(os.path.join(self._path, filename))
             for filename in os.listdir(self._path)] or [0])
         return time.strftime("%a, %d %b %Y %H:%M:%S +0000", time.gmtime(last))
+
+    @property
+    @contextmanager
+    def props(self):
+        # On enter
+        properties = {}
+        if os.path.exists(self._props_path):
+            with open(self._props_path) as prop_file:
+                properties.update(json.load(prop_file))
+        old_properties = properties.copy()
+        yield properties
+        # On exit
+        if os.path.exists(self._props_path):
+          self._create_dirs()
+          if old_properties != properties:
+              with open(self._props_path, "w") as prop_file:
+                  json.dump(properties, prop_file)