giomodule: add a new "get default" function

_gio_module_get_default() is a very convenient function for modules
implementing a singleton -- it finds the default module by priority
subject to override by a given environment variable name, instantiates
it, and caches the instance for future calls.  It also has the ability
to query instances for being 'active' using a callback.

It doesn't work very well for non-singletons (like file monitors).

Add a new function _gio_module_get_default_type() that skips the
instantiation, returning the GType instead.  As a replacement for the
'active' callback, a vtable offset can be given for a virtual function
to use to query if a particular backend is supported.

https://bugzilla.gnome.org/show_bug.cgi?id=592211

gio/giomodule-priv.h

@@ -35,6 +35,10 @@ gpointer _g_io_module_get_default (const gchar         *extension_point,
 				   const gchar         *envvar,
 				   GIOModuleVerifyFunc  verify_func);
 
+GType    _g_io_module_get_default_type (const gchar *extension_point,
+                                        const gchar *envvar,
+                                        guint        is_supported_offset);
+
 #ifdef G_PLATFORM_WIN32
 void *_g_io_win32_get_module (void);
 #endif