quassel: Add a package option

[uwap]

Motivation for this change

There are packages for quassel with qt4 and qt5 because quassel and KDE don't work together with qt5. For a server application this is irrelevant if you have a server that deploys a quassel daemon. A qt5 version can has advantages so it should be up to the user what package should be used.

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
  • OS X
  • Linux
• 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.

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[mention-bot]

@uwap, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Phreedom, @Shados and @peti to be potential reviewers.

[Profpatsch]

Thanks!