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

docs/reference/gobject/gobject-sections.txt

@@ -480,6 +480,7 @@ g_param_spec_ref
 g_param_spec_unref
 g_param_spec_sink
 g_param_spec_ref_sink
+g_param_spec_get_default_value
 g_param_value_set_default
 g_param_value_defaults
 g_param_value_validate