manifest: switch clutter-gst to branch 2.0
manifest: switch clutter-gst to branch 2.0

Significant developpement breaking API is ongoing on clutter-gst's
master branch. Master has been branch to clutter-gst-2.0 to support
existing apps.

chergert/gnome-continuous,chergert/gnome-continuous,GNOME/gnome-ostree,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,djdeath/gnome-ostree,vrutkovs/vrutkovs.github.io,GNOME/gnome-ostree,djdeath/gnome-ostree,djdeath/gnome-ostree,GNOME/gnome-ostree