Feat: nested spaces

[sajattack]

Forgive me if this has already been mentioned but I did a quick look through your issue board and didn't see it.

Element supports spaces within spaces - which comes in handy especially if you're bridging discords to matrix.

It would be nice if these nested spaces could be viewed with proper hierarchy.

Allow me to illustrate:

[Ramblurr]

Nested spaces can be navigated with hjkl + enter, but there's no easy way to go back up a level, which would be handy.

Alternatively I guess a tree view would be nicer (though more complicated to implement).

[VAWVAW]

Since a space doesn't always have an unambiguous parent there isn't always a simple answer as to what "up" should be. You could define "up" as "where I came from" but that can already be achieved using the jump list (shortcut <C-O>).