timeshift: init at 22.06.1

[ShamrockLee]

Motivation for this change

Timeshift is commonly seen in Debian-based distros, and is enabled by default on Ubuntu and Linux Mint.

If applied, users will be able to restore their broken distros from timeshift snapshots.

closes #89348

I'm not sure if it's useful for NixOS users, since we already have a powerful package manager that does in-place snapshots and rollback for us. (There is a comment in the package request about the possible use case when updating to a new version.)
Nevertheless, nix users on other distros may find it helpful to be able to install this from Nix and to restore inside that distro or from NixOS.

To-do

• The "periodic snapshot" functionality requires crontab access from superuser. (this can be disabled)
• timeshift-gtk expects /etc/timeshift/timeshift.json or /etc/timeshift/default.json to exist before it starts. I'm not sure how other packages manage that. (Solved by patching src/Core/Main.vala with substituteInPlace)
• The snapshot creation/restore functionality has not tested yet. It would be great to add corresponding NixOS tests.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[SuperSandro2000]

Please format the files with nixpkgs-fmt.

[dotlambda]

We might want to switch to https://github.com/linuxmint/timeshift.

[SuperSandro2000]

I am not super sure about all the wrapping and seperation but should be fine.

[ShamrockLee]

@SuperSandro2000 There are quite a few changes regarding the postPatch phase. Please take a look before merging.