XFA -- Display text content

- display xhtml;
  - allow spaces in xhtml (xfa-spacerun:yes);
  - support column layout;
  - fix some border issues.

src/core/xfa/xfa_object.js

@@ -19,6 +19,7 @@ import { NamespaceIds } from "./namespaces.js";
 
 // We use these symbols to avoid name conflict between tags
 // and properties/methods names.
+const $acceptWhitespace = Symbol();
 const $appendChild = Symbol();
 const $childrenToHTML = Symbol();
 const $clean = Symbol();
@@ -131,6 +132,10 @@ class XFAObject {
     );
   }
 
+  [$acceptWhitespace]() {
+    return false;
+  }
+
   [$setId](ids) {
     if (this.id && this[$namespaceId] === NamespaceIds.template.id) {
       ids.set(this.id, this);
@@ -805,6 +810,7 @@ class Option10 extends IntegerObject {
 }
 
 export {
+  $acceptWhitespace,
   $appendChild,
   $childrenToHTML,
   $clean,