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.

TODO.md

@@ -2,7 +2,6 @@
 - Invited → Accept/Deny dialog
 - Keep the room header name and topic updated
 - Merge login page
-- Show actual display name for AccountDelegate
 
 - When inviting someone to direct chat, room is "Empty room" until accepted,
   it should be the peer's display name instead.
@@ -19,8 +18,6 @@
 
 - Migrate more JS functions to their own files
 
-- Accept room\_id arg for getUser
-
 - Set Qt parents for all QObject
 
 - `<pre>` scrollbar on overflow
@@ -39,3 +36,5 @@
 - ![A picture](https://picsum.photos/256/256) not clickable?
 
 - On sync, check messages API, if a limited sync timeline was received
+
+- Graphic bug when resizing window vertically for side pane?