gparam: Introduce nick and blurb setters

Instead of having them on GProperty, let's add the static nick and blurb
setters where they belong, i.e. GParamSpec.
gobject: Add GProperty

Dealing with GParamSpec is tedious and less efficient than necessary;
property definitions should be able to either directly access a struct
field or specify the accessors pair that control a property. On top of
that, most of the property and accessor definition can be autogenerated
from simple pre-processor directives.

So, here's to you GProperty.

GProperty is a GParamSpec sub-class that encapsulates all the types
inside a single, opaque structure (to avoid leaking out implementation
details); a GProperty can access the structure member holding the value
of the property, or invoke the accessor functions passed to its
constructor. Type safety is maintained through the GType system and
without having to use GValue.

Along with GProperty, this patch introduces a series of macros for
automating the declaration and definition of property accessor functions,
and for automating the collection and lcopy of values without going
through GValue.

GObject will recognize whether a GParamSpec used to set or get
a property is really a GProperty, and thus will shortcircuit most of
the GValue-based marshalled code, preferring the direct C function
and direct argument collection instead of boxing/unboxing of GValues.

https://bugzilla.gnome.org/show_bug.cgi?id=648526
property: Add direct access to generated getters

This is mostly a WIP/proof of concept patch on top of GProperty. the
numbers from the performance test case say that we don't gain much, if
at all, by checking if the property maps to a direct field offset, and
we get the pointer to the struct member.