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/Makefile.am

@@ -288,6 +288,8 @@ libgio_2_0_la_SOURCES =		\
 	gdrive.c 		\
 	gdummyfile.h 		\
 	gdummyfile.c 		\
+	gdummyproxyresolver.c	\
+	gdummyproxyresolver.h	\
 	gemblem.h 		\
 	gemblem.c 		\
 	gemblemedicon.h		\
@@ -331,6 +333,7 @@ libgio_2_0_la_SOURCES =		\
 	gpermission.c 		\
 	gpollfilemonitor.c 	\
 	gpollfilemonitor.h 	\
+	gproxyresolver.c	\
 	gresolver.c		\
 	gseekable.c 		\
 	gsimpleasyncresult.c 	\
@@ -478,6 +481,7 @@ gio_headers =			\
 	gnetworkservice.h	\
 	goutputstream.h 	\
 	gpermission.h 		\
+	gproxyresolver.h	\
 	gresolver.h		\
 	gseekable.h 		\
 	gsimpleasyncresult.h 	\