Create a top-level "third_party" directory

This directory now contains third_party software that is vendored into
the Servo source tree. The idea is that it would eventually hold
webrender and other crates from mozilla-central as well with a standard
patch management approach for each.

components/script/dom/bindings/codegen/parser/callback-location.patch

@@ -1,20 +0,0 @@
---- WebIDL.py
-+++ WebIDL.py
-@@ -2283,7 +2283,7 @@ class IDLUnresolvedType(IDLType):
-             return typedefType.complete(scope).withExtendedAttributes(self.extraTypeAttributes)
-         elif obj.isCallback() and not obj.isInterface():
-             assert self.name.name == obj.identifier.name
--            return IDLCallbackType(self.location, obj)
-+            return IDLCallbackType(obj.location, obj)
- 
-         name = self.name.resolve(scope, None)
-         return IDLWrapperType(self.location, obj)
-@@ -6854,7 +6854,7 @@ class Parser(Tokenizer):
-                     type = IDLTypedefType(self.getLocation(p, 1), obj.innerType,
-                                           obj.identifier.name)
-                 elif obj.isCallback() and not obj.isInterface():
--                    type = IDLCallbackType(self.getLocation(p, 1), obj)
-+                    type = IDLCallbackType(obj.location, obj)
-                 else:
-                     type = IDLWrapperType(self.getLocation(p, 1), p[1])
-                 p[0] = self.handleNullable(type, p[2])