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/SendBox.qml

@@ -20,7 +20,7 @@ Rectangle {
 
         Base.Avatar {
             id: "avatar"
-            name: Backend.getUser(chatPage.user_id).display_name
+            name: Backend.getUserDisplayName(chatPage.user_id)
             dimmension: root.Layout.minimumHeight
             //visible: textArea.text === ""
             visible: textArea.height <= root.Layout.minimumHeight