Add global config file support for SharedOptions

[remique]

This Pull Request fixes/closes #2140.

It changes the following:

• If gitui fails to find a local config file, it falls back to global options under $GITUI_HOME/gitui.ron

I followed the checklist:

• [ ] I added unittests
• [x] I ran make check without errors
• [x] I tested the overall application
• [ ] I added an appropriate item to the changelog

Also:

1. We should probably document this somewhere? If a person sets a global config and then sees that it does not change a thing in his gitui (due to the local config), they might be confused.
2. I have renamed gitui file to gitui.ron so that both global and local configs match. I hope that's okay.

[remique]

Oh, I was surprised by builds not passing but I see that clippy::empty_docs has been added in 1.78.0.