Remember open/closed folder state across sessions

### Is your feature request related to a problem? Please describe.
Sometimes, I do commits with quite a bunch of changes, and I stage some parts one-by-one, closing the irrelevant folders in the file tree, while keeping the ones with the changes I'm focusing on open. However, this isn't saved when quitting lazygit, so each time I reopen it I have to close some folders in the file tree.

### Describe the solution you'd like
I would like to have a config option for lazygit to save the open or closed state of each folder in the working tree before quitting, which can then be restored next time.

Start by tracing how lazygit builds and updates the working-tree file tree, then inspect existing configuration and shutdown/session persistence behavior. Done means a configuration option saves each folder's open or closed state when lazygit exits and restores it when the application is reopened.

Written by the indexing model from the issue text.