Make clutter-gst for GStreamer 1.0 parallel installable

We need to be able to ship both the GStreamer 0.10 and the GStreamer
1.0 versions of clutter-gst, and have them not clash.

We used to install clutter-gst in a clutter-1.0 directory, but only
the GStreamer API version changed, not the clutter one, so we need to
move the includes to $(includedir)/clutter-gst-2.0/

https://bugzilla.gnome.org/show_bug.cgi?id=674077