Show spaces horizontal bar

### Summary

This merge request adds a horizontal bar at top of room list, which shows spaces. By clicking on a space, user can filter out rooms belonging only to that specific space.

### Pending/ Help needed

#### Segfault when loading active connection on startup

Refer `void SortFilterRoomListModel::cacheSpaceHierarchy()` ([link](8c372800d7 (b969e462c30df43ef3714ea441948d8d8027f6a0_117_126))) in `src/sortfilterroomlistmodel.cpp`.

On [line 129](8c372800d7 (b969e462c30df43ef3714ea441948d8d8027f6a0_117_129)), I have called `connection->allRooms()`, which segfaults if the active connection hasn't been loaded yet. Is there a way to ensure that `Controller::instance().activeConnection()` on line 128 waits till connection is loaded?

#### Avatars

Avatars on space horizontal bar aren't aligned to vertical middle. I'll need help with that. 

Using the code below doesn't help with padding

```qml
delegate: QQC2.Control {
    topPadding: 10
    contentItem: Kirigami.Avatar { ..... }
}
```

This complains about uninitialized properties in `Kirigami.Avatar`. (`id`, `currentRoom`, `avatar`, `index` are properties utilized during run time)

After we get around these two issue, this MR will be ready from my side.

src/roomlistmodel.cpp

@@ -408,6 +408,10 @@ QVariant RoomListModel::data(const QModelIndex &index, int role) const
     if (role == IdRole) {
         return room->id();
     }
+    if (role == IsSpaceRole) {
+        return room->isSpace();
+    }
+
     return QVariant();
 }
 
@@ -439,6 +443,8 @@ QHash<int, QByteArray> RoomListModel::roleNames() const
     roles[CurrentRoomRole] = "currentRoom";
     roles[CategoryVisibleRole] = "categoryVisible";
     roles[SubtitleTextRole] = "subtitleText";
+    roles[IsSpaceRole] = "isSpace";
+    roles[IdRole] = "id";
     return roles;
 }