Proper display name retrieval implementation

For any name not found in rooms data, rely on new
nio.HttpClient.get_displayname() function to get and cache it,
e.g. for our own name if no room is joined and past events from users
who left the room.

@futurize now returns PyQtFuture objects, wrapper for the
concurrent.futures.Future objects that can be used from QML,
to ensure name retrieval does not block the GUI.

harmonyqml/components/chat/MessageDelegate.qml

@@ -21,8 +21,8 @@ Column {
     readonly property bool isUndecryptableEvent:
         type === "OlmEvent" || type === "MegolmEvent"
 
-    readonly property string displayName:
-        Backend.getUser(dict.sender).display_name
+    readonly property var displayName:
+        Backend.getUserDisplayName(dict.sender)
 
     readonly property bool isOwn:
         chatPage.user_id === dict.sender