Revert "Use xfce4-mixer instead of Gnome mixer"

This reverts commit 4532e499e951377947544f07ab70058e0d67e5f5.

Conflicts:
	configuration/etc/xdg/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml
Install appmenu-gtk to prevent DBus errors