GParamSpec: add g_param_spec_get_default_value()

The way of getting the default value out of a GParamSpec is to allocate
a GValue, initialise it, then call g_param_spec_set_default() to set the
default value into that GValue.

This is exactly how we handle setting the default value for all of the
construct properties that were not explicitly passed to g_object_new().

Instead of doing the alloc/init/store on all construct properties on
every call to g_object_new(), we can cache those GValues in the private
data of the GParamSpec itself and reuse them.

This patch does not actually make that change to g_object_new() yet, but
it adds the API to GParamSpec so that a future patch to GObject can make
the change.

https://bugzilla.gnome.org/show_bug.cgi?id=698056

gobject/gparam.h

@@ -336,6 +336,8 @@ void           g_value_take_param               (GValue        *value,
 GLIB_DEPRECATED_FOR(g_value_take_param)
 void           g_value_set_param_take_ownership (GValue        *value,
                                                  GParamSpec    *param);
+GLIB_AVAILABLE_IN_2_36
+const GValue *  g_param_spec_get_default_value  (GParamSpec     *param);
 
 /* --- convenience functions --- */
 typedef struct _GParamSpecTypeInfo GParamSpecTypeInfo;
@@ -421,7 +423,6 @@ GParamSpec**	g_param_spec_pool_list		(GParamSpecPool	*pool,
 						 guint		*n_pspecs_p);
 
 
-
 /* contracts:
  *
  * gboolean value_validate (GParamSpec *pspec,