Implemented proxy-resolver extension point

This extension point allow extending GLib with library like LibProxy that
interprets system proxy settings and finds the appropriate configuration
based on the type of connection being made.

Reviewed-by: Dan Winship <danw@gnome.org>

gio/giotypes.h

@@ -198,6 +198,15 @@ typedef struct _GThreadedSocketService                      GThreadedSocketServi
 typedef struct _GThemedIcon                   GThemedIcon;
 typedef struct _GVfs                          GVfs; /* Dummy typedef */
 
+/**
+ * GProxyResolver:
+ *
+ * A helper class to enumerate proxies base on URI.
+ *
+ * Since: 2.26
+ **/
+typedef struct _GProxyResolver                GProxyResolver;
+
 /**
  * GVolume:
  *