Stop dithering over GPrivate

Take out the half-private g_private_init() stuff and replace it with a
G_PRIVATE_INIT macro that allows specifying a GDestroyNotify.

Expose the GPrivate structure in a public header.

Add a g_private_replace() to (sort of) match the functionality of
g_static_mutex_set().

Improve the documentation.

Deprecate g_private_new().

docs/reference/glib/glib-sections.txt

@@ -691,9 +691,13 @@ g_cond_broadcast
 
 <SUBSECTION>
 GPrivate
-g_private_new
+G_PRIVATE_INIT
 g_private_get
 g_private_set
+g_private_replace
+
+<SUBSECTION>
+g_private_new
 
 <SUBSECTION>
 GStaticPrivate