PulseAudio no more required

Upgrading to mate-settings-daemon-1.1.1, we readded support to gstreamer (instead of pulseaudio) for media (volume) keys. So, now MATE dont requires PulseAudio.

Now there are two packages that provides mate-settings-daemon:

• mate-settings-daemon-gstreamer (default): uses gstreamer to provides media keys.
• mate-settings-daemon-pulse: uses pulseaudio to provides media keys.