Show untracked files in stash

**Is your feature request related to a problem? Please describe.**
Sometimes I stash a bunch of untracked files, to get them out of the way. When viewing such a stash in lazygit, I unfortunately can't see, if there are untracked ones and what they contain.

**Describe the solution you'd like**
If I click on a stash, it could change to a view of indexed and untracked files, like I can do on commits.

**Describe alternatives you've considered**
An alternative would be to just display the diff in the Stash view on the right.