[CPML] Added (still unused) primitive VT

[CPML] Added (still unused) primitive VT

CpmlPrimitiveClass is the abstract class to be implemented by all the
primitive types. The next step will be to fill the virtual table of
every primitive and collect them in some way for further processing by
the CpmlPrimitive public APIs.

The actual idea is to instantiate different CpmlPrimitiveClass, once for
every primitive type. Once done, the CpmlPrimitive abstract methods will
be able to access the underlying implementations from a single entry
point. A bunch of hidden initialization functions will be needed as well
(similar to what the g_..._get_type() functions do the GObject world).

Started issue #30: http://dev.entidi.com/p/adg/issues/30/