View subspaces while the Spaces-bar is minimised.

#### Is your suggestion related to a problem? Please describe.
When viewing subspaces in a space, the space-bar automatically expands, and takes up a huge amounts of valuable screen real-estate. This makes Element take up a lot more space than what's necessary, and it looks very clunky, aseptically on small screens.

#### Describe the solution you'd like.
You should be able to view sub-spaces while the space-bar is minimised.

#### Describe alternatives you've considered.
It could be solved like this:
My inspiration is from Discords server folders:

#### Additional context
This does mean that you won't be able so see the names of the subspaces unless you hover over them. But if it's a space you use a lot, it won't matter, since you know it well. And this won't be an issue if vector-im/element-web#17378 is added, since it will show the subspaces in the room list anyway.