Add glib__private__() API to share between glib,gio; port GWakeup to it
Historically we've added random symbols to the public API with warnings
that they're private; examples are:
glib_gettext(), glib_pgettext()
g_thread_functions_for_glib_use, g_thread_use_default_impl, etc.
And we almost added "GWakeup" to public API just to share between glib and
gio.
This new glib__private__() API exports a hidden vtable, and adds a macro
GLIB_PRIVATE_CALL() that makes it generally convenient to use.
This adds an extremely tiny cost for the double indirection; but it has
the benefit that we don't need to either:
1) compile the code into both glib and gio (like GWakeup), with the
inefficiency that implies.
2) Export a "do not use this" symbol; the serious problem with this is
that someone CAN use it pretty easily. Particularly if we document
it. It's far, far harder to peek into a structure without a public
header file.
https://bugzilla.gnome.org/show_bug.cgi?id=657992