### 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.
