Allow pinning child spaces to top-level

### Your use case

#### What would you like to do?
I would like to be able to show a child space that I am joined to in the top-level of the space bar (on the left side of the screen).

#### Why would you like to do it?
Currently, the only apparent way to access a child space that I am joined to via mouse clicks is to drill into it from its parent spaces. For deeply-nested child spaces, this can be cumbersome.

#### How would you like to achieve it?
With a right-click / context menu item on a child space, or an entry in a child space's Preferences menu, that contains a toggle for whether to pin it to the top-level of the space bar. This toggle would appear only for spaces that have a parent space that you're joined to.

### Have you considered any alternatives?

A workaround is to leave all of the space's parents, which effectively makes the target space a top-level one. But, I want to remain joined to the parent spaces.

An alternative is to use the search menu to find & enter the space, but that requires remembering the name of the space, or to have searched for it recently enough for it to appear in recent searches before typing its name.

### Additional context

_No response_