Let me nest custom sources

[Mosnar]

I'd like to be able to create custom sources under existing sources - this helps keep the element index sidebar from getting too cluttered. For example:

Blog Posts (section)
-> Posts without images (custom source)
Events (section)
-> 2022 Events (custom source)
-> 2021 Events (custom source)
-> 2020 Events (custom source)

[kylecotter]

We've resorted to some unicode arrow characters to fake this, but would love to see this built in too!

[Mosnar]

I ended up using these characters for now: ├ └

This is the result: