Introduce automatic backups

[mxgrey]

Before proceeding, is there an existing issue or discussion for this?

• I have done a search for similar issues and discussions.

Description

It would be good for the application to periodically save backups while the user is working. There are many ways we could approach this. Here are a few ideas:

1. A single hidden backup file next to the last saved file, e.g. my_project.site.json would be accompanies by .my_project.site.json.backup`
2. Each site file gets a directory inside of $HOME/.cache/rmf_site_editor/backups that keeps the N latest backups with timestamps where N can be set by the user

Some settings to consider providing:

• Notify users when they are loading a file whose associated backup is newer than the loaded file, and allow the user to load the backup instead
• Allow the user to see the latest backup(s) with their timestamps, and to ask to switch to those backups
• Allow the user to pin a backup so that it gets retained indefinitely