gdbus: properly escape introspection annotations

Make sure we escape any special characters that are found in annotation
names or values to avoid emitting a malformed XML document in response
to an Introspect call.

https://bugzilla.gnome.org/show_bug.cgi?id=721796
gdbus-introspection test: test escaping attributes

Make sure attribute values are properly escaped in generated XML.

https://bugzilla.gnome.org/show_bug.cgi?id=721796