Nixpkgs: Media keys don't work with xfce4-pulseaudio-plugin in xfce

[worldofpeace]

Describe the bug
Moved over from discourse https://discourse.nixos.org/t/media-keys-not-working-with-xfce4/5846.

To Reproduce
Steps to reproduce the behavior:

1. Use Xfce and enable pulseaudio
2. Try to use media keys

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/media-keys-not-working-with-xfce4/5846/6

[JeffLabonte]

Xfce4-14 on 19.09 doesn't allow me to use the volume key. I need to run the old version of Xfce if I want to have at least that working.

[worldofpeace]

This fixes that, https://github.com/NixOS/nixpkgs/pull/79844. Will backport to 19.09 somehow.

[vcunat]

Seems all done.