Provide a way to temporarily hide "custom patch" panel

**Is your feature request related to a problem? Please describe.**
When I am selecting lines/chunks to add to a patch, I have this problem where it is difficult to see the full code I am selecting because the "custom patch" panel is taking up too much space. For example, here I have a chunk selected, but I have to scroll to see the full chunk:


**Describe the solution you'd like**
I would prefer a workflow where I'm focused on selecting lines/chunks and then, when I'm done, switching to another view which just shows the custom patch. This is similar to what I would do outside of `lazygit`—I would run `git add --patch` and then, when I'm done, run `git diff --cached` to review the staged patch before committing. If I need to make more changes I can jump back to `git add --patch`.

**Describe alternatives you've considered**

**Additional context**
#749 would also help with this