This adds a priority property to all ClutterActorMetas. The
ClutterMetaGroup keeps the list sorted so that higher priority metas
remain at the beginning of the list. The priority is a signed integer
with the default as zero. An actor meta can therefore be put before
all default metas with a positive number, or after with a negative
number.

There are constants to set an 'internal' priority. The intention is
that applications wouldn't be allowed to use these values so that we
can keep special internal metas to that are before or after all
application metas.

The property isn't a real GObject property because for now it is
completely internal and only used to implement the 'transparency'
property of ClutterActor. ClutterMetaGroup doesn't currently resort
the list if the property changes so if we wanted to make it public we
should either make it construct-only or make the meta group listen for
changes on the property and resort accordingly.

The methods in ClutterActor that get the list of metas now use a new
function that filters out internal metas from the meta
group. Similarly for clearing the metas, the internal metas are left
in.