Plug-ins: Speed up initialization by avoiding their loading on startup.

	* tools/extra_defs_gen/generate_plugin_gmmproc_file.cc
	(generate_hg_file): Use the new _NO_WRAP_INIT_REGISTRATION macro to
	signal that the plug-ins should not be registered on startup by the
	wrap_init() function.  Also use the new _CUSTOM_WRAP_FUNCTION in order
	to implement a custom Glib::wrap() function for plug-ins that ensures
	the registration of the wrap_new() function before calling
	Glib::wrap_auto() to wrap a C object.
	(generate_ccg_file): Modify the generation of the plug-in *_get_type()
	functions so that when an object of the plug-in type is first created,
	the plug-in's wrap_new() function is registered with the wrapping
	system using the type obtained when the plug-in is first loaded.  Also
	generate a custom Glib::wrap() function that ensures the registration
	of the plug-in's wrap_new() function before calling Glib::wrap_auto().
	This ensures that even if there is a C type of the plug-in, that even
	that object can be properly wrapped.

	Bug #684006.