XFA - Implement usehref support

- attribute 'use' was already implemented but not usehref
  - in general, usehref should make reference to current document
  - add support for SOM expressions in use and usehref to search a node.
  - get prototype for all nodes if any.

src/core/xfa/template.js

@@ -3866,9 +3866,6 @@ class Subform extends XFAObject {
   }
 
   [$toHTML](availableSpace) {
-    if (this.name === "helpText") {
-      return HTMLResult.EMPTY;
-    }
     if (this[$extra] && this[$extra].afterBreakAfter) {
       const ret = this[$extra].afterBreakAfter;
       delete this[$extra];
@@ -3890,8 +3887,6 @@ class Subform extends XFAObject {
       );
     }
 
-    // TODO: implement usehref (probably in bind.js).
-
     // TODO: incomplete.
     fixDimensions(this);
     const children = [];