Log Message:
Rename gstreamer, gst-plugins and gst-plugins-* to gstreamer0.8,
gst-plugins0.8 and gst-plugins0.8-* respectively.  This is to leave room
for gstreamer 0.10 (which will be added with versioned names too, to avoid
annoying renames in the future).

While here, do some changes:
- Make gstreamer0.8 use the alternatives system to provide the unversioned
  gst-* utilities.
- Rename all internal variables to have 0.8 in them to make it clear which
  package they belong to.