Improve room past events loading

- Trigger when room is shown if there's not enough messages to fill the
  list height

- Trigger whenever user is scrolling before a certain point, instead of
  when dragging is released/scrolling stopped and the top edge is hit

- Prevent multiple load requests at same time

- Keep a set of fully loaded rooms, don't request anymore history
  if a room is fully loaded

TODO.md

@@ -30,3 +30,7 @@
 - Push instead of replacing in stack view
 
 - QQuickImageProvider, matrix preview API
+
+- Spinner when loading past room events or images
+
+- nio: org.matrix.room.preview\_urls, m.room.aliases