feature!: Improve and modify Auto-remove

[buhtz]

Blocked by and follow-up of #1976
Meta issue #1945

Problem

The Auto-remove feature has potential for improvement. It is complex in some details. And the code is not organized in a way to create isolated unit tests.

Goals

1. Refactor the code.
2. Add unit tests.
3. Modify the behavior with revising existing rules, maybe adding new rules and/or removing rules.
4. Modify user manual accordingly.

Details about modifications considered

• Term "snapshot" to "backup" (Issue ???)
• Remove? "Remove if snapshot is older than N years/months/weeks." (ref)
  • Because of that modify: "Keep one snapshot per year for all years" from QLabel to a modifyable value.
• Maybe: Time-independent rule: "No matter what, keep the NN last backups" (ref)
• Think about rules respecting each other and not overwritting. Example "Keep one monthly backup for 12 months" plus "Always keep the last 10 daily backups"
• Inode/space rules: Remove them maybe.
• Separate feature: pre-warning if user-defined threshold of space/inode limit is reached.
• Ask user "ceperman" for review (ref).