Move ShowAuthorRole to MessageFilterModel

Only there can it be calculated correctly.
Fixes #632

src/models/messagefiltermodel.h

@@ -34,6 +34,7 @@ public:
         StateEventsRole, /**< List of state events in the aggregated state. */
         AuthorListRole, /**< List of the first 5 unique authors of the aggregated state event. */
         ExcessAuthorsRole, /**< The number of unique authors beyond the first 5. */
+        ShowAuthorRole, /**< Whether the author (name and avatar) should be shown at this message. */
         LastRole, // Keep this last
     };