* clutter/clutter-behaviour-path.h:
* clutter/clutter-behaviour-path.c: Add "since" strings; add
a "knot" property which can be used to append a knot to the path;
fix apidoc.
* clutter/clutter-alpha.c: Add "since" strings.
* clutter/clutter-feature.c: Add apidoc.
* clutter/clutter-behaviour-opacity.c: Add the "opacity-start"
and "opacity-end" properties; rewrite constructor to use them.
