Make the files view filtering setting persistently configurable

### Is your feature request related to a problem? Please describe.
I almost always prefer to show only tracked files in the files view. However, this cannot be configured persistently in Config.yml, only at runtime.

### Describe the solution you'd like
Add a setting to Config.yml so that lazygit can start up with my preferred filter.

### Describe alternatives you've considered
The only alternative is just pressing `C-b t` every time I start up lazygit, as far as I can tell.

### Additional context
The functionality was added in #4226 and #4230.

I have not submitted this as a PR as per the contributing instructions. However, I do have this [implemented and working](https://github.com/jesseduffield/lazygit/compare/master...durka:lazygit:feat/files-view-filtering-config) (including hot-reloading, but I need a bit of advice on the codebase to make that cleaner).