XFA - Add a parser for XFA files

- the parser is base on a class extending XMLParserBase
 - it handle xml namespaces:
   * each namespace is assocated with a builder
   * builder builds nodes belonging to the namespace
   * when a node is inserted in the parent namespace compatibility is checked (if required)
 - to avoid name collision between xml names and object properties, use Symbol.

gulpfile.js

@@ -1431,7 +1431,7 @@ function buildLib(defines, dir) {
   return merge([
     gulp.src(
       [
-        "src/{core,display,shared}/*.js",
+        "src/{core,display,shared}/**/*.js",
         "!src/shared/{cffStandardStrings,fonts_utils}.js",
         "src/{pdf,pdf.worker}.js",
       ],