Rewrite sound applet as Vala plugin

This actually introduces the "Status Area" applet, which can consist of
indicators designed for the Budgie Desktop (not to be confused with Ubuntu's
indicators, GNOME's indicators, or even car indicators.)
As we begin to add more icons to the applet we can introduce a combined menu
to control all of them easily, as opposed to one menu per icon (much like
how GNOME Shell handles it. Kudos, GNOME.)