component file sharing: comments metadata:

new <comments> element is added to file metadata, it contains the URL to the comments virtual node and the count of comments (this way client knows if it make sense to request comments or not).
Fixed triggers in plugin XEP-0264 (return value was missing).
New trigger in plugin XEP-0329 to allow component to add metadata (used here for comments url).

src/plugins/plugin_xep_0264.py

@@ -91,6 +91,7 @@ class XEP_0264(object):
             width, height = thumbnail['size']
             thumbnail_elt['width'] = unicode(width)
             thumbnail_elt['height'] = unicode(height)
+        return True
 
     def _getFileThumbnails(self, file_elt, file_data):
         thumbnails = []
@@ -113,6 +114,7 @@ class XEP_0264(object):
 
         if thumbnails:
             file_data.setdefault('extra', {})[C.KEY_THUMBNAILS] = thumbnails
+        return True
 
     ## thumbnails generation ##