xgl.py: add Extension

core and XGL_WSI_X11 are both Extensions.  Add

  extensions, a list of all extensions
  headers, a list of C headers of all extensions
  protos, a list of C prototypes of all extensions
diff --git a/glave-generate.py b/glave-generate.py
index b7dfb06..fcb0a7a 100755
--- a/glave-generate.py
+++ b/glave-generate.py
@@ -32,12 +32,10 @@
 class Subcommand(object):
     def __init__(self, argv):
         self.argv = argv
-        self.protos = ()
-        self.headers = ()
+        self.headers = xgl.headers
+        self.protos = xgl.protos
 
     def run(self):
-        self.protos = xgl.core + xgl.ext_wsi_x11
-        self.headers = xgl.core_headers + xgl.ext_wsi_x11_headers
         print(self.generate())
 
     def generate(self):