Rooms and threads fixes

- Fix roomList height again, now based on model.count().
  All delegates are assumed to be the same height

- Properly update room list when a room is joined or left

- Catch exceptions happening in threads (futures), which previously
  passed silently

- Show "Empty room?" as "<i>Empty Room</i>" + gray [?] avatar

harmonyqml/backend/model/items.py

@@ -17,7 +17,7 @@ class User(NamedTuple):
 
 class Room(NamedTuple):
     room_id:                    str
-    display_name:               str
+    display_name:               Optional[str]
     description:                str           = ""
     unread_messages:            int           = 0
     presence:                   Presence      = Presence.none

harmonyqml/backend/model/list_model.py

@@ -129,7 +129,7 @@ class ListModel(QAbstractListModel):
 
     @pyqtSlot(int, list)
     def set(self, index: int, value: NewValue) -> None:
-        qidx              = QAbstractListModel.index(index, 0)
+        qidx              = QAbstractListModel.index(self, index, 0)
         value             = self._convert_new_value(value)
         self._list[index] = value
         self.dataChanged.emit(qidx, qidx, self.roleNames())
@@ -139,7 +139,7 @@ class ListModel(QAbstractListModel):
     @pyqtSlot(int, str, "QVariant")
     def setProperty(self, index: int, prop: str, value: Any) -> None:
         self._list[index][self._roles.index(prop)] = value
-        qidx = QAbstractListModel.index(index, 0)
+        qidx = QAbstractListModel.index(self, index, 0)
         self.dataChanged.emit(qidx, qidx, self.roleNames())
         self._update_count += 1