Mainvolume

With new PulseAudio and settings Mainvolume module for volume control is again in use. If/when route volumes are enabled in Nemo, Mainvolume is better entry point for controlling volumes.
So partially reverted "pulseaudio client" commit, which changes volume-control to connect to PulseAudio via libpulse. Also updated dbus-socket location to search for XDG_RUNTIME_DIR first, before falling back to old implementation.