[IMP] qweb: allow xml documents as template definition

Before this commit, the addTemplates method only accepts a string.  But
whenever we perform a $.get call, we get a parsed xml document. So, to
make Owl more convenient to use in that situation, we simply accept xml
documents in addition to strings.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "owl",
-  "version": "0.18.0",
+  "version": "0.18.1",
   "description": "Odoo Web Library (OWL)",
   "main": "src/index.ts",
   "scripts": {

src/qweb/qweb.ts

@@ -207,12 +207,12 @@ export class QWeb extends EventBus {
   }
 
   /**
-   * Load templates from a xml (as a string).  This will look up for the first
-   * <templates> tag, and will consider each child of this as a template, with
-   * the name given by the t-name attribute.
+   * Load templates from a xml (as a string or xml document).  This will look up
+   * for the first <templates> tag, and will consider each child of this as a
+   * template, with the name given by the t-name attribute.
    */
-  addTemplates(xmlstr: string) {
-    const doc = parseXML(xmlstr);
+  addTemplates(xmlstr: string | Document) {
+    const doc = typeof xmlstr === 'string' ? parseXML(xmlstr) : xmlstr;
     const templates = doc.getElementsByTagName("templates")[0];
     if (!templates) {
       return;