Restructure extension

This changes all references to the old name and makes it compatible with
version 3.12.

Other changes:
  *) Made aggregate (status) menu the default option (to be less disruptive)
  *) Cleaned up the code a bit
  *) Fixed error when initializing the mixer
  *) Added an option not to remove the master volume mixer when using
     the separate menu
  *) Monkey patched the slider to allow middle button mute events to
     bubble
  *) Added GPLv2 license. I'm not a laywer, but as you have to check a
     box before uploading an extension to GNOME this might be OK.