feat: Implement collapsing nested subentries

[jamesbtan]

This is with regards to issue #23 .

The main feature set is implemented, with a brief demo here.

As of now I am just always adding 2em of space for the button before each row, regardless of whether there will be any buttons there or not. Mainly I was trying to preserve the alignment of elements in the same indentation whether they have a button or not, but if anyone wants to suggest a better way to format this that would be welcome.

[jamesbtan]

I just realized it is breaking the formatting of the edit row view. I can try to come up with a solution later.

[Ellpeck]

Thanks so much for your work!! I'll review this when I next take some time to work on the plugin :)

[Ellpeck]

Looks good! I'll merge this when I next work on the plugin again :) Thanks so much!!