Motivation for this change

The release notes for Nixos 17.09 say: "Touchpad support should now be enabled through libinput as synaptics is now deprecated. See the option services.xserver.libinput.enable."
Because a libinput.twoFingerScroll option doesn't exist, I chose an equivalent boolean option as new example libinput.tapping.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.