* clutter/Makefile.am: Add clutter-units.c
* clutter/clutter-units.[ch]: Add a ClutterUnit fundamental
type, and wrappers for storing it into GValues; also add a
GParamSpec subclass for defining parameters holding ClutterUnit
values. This allows creating GObject properties using units.
* doc/reference/clutter-sections.txt: Document the newly
added API.