[IMP] qweb: expose translatable attributes

This commit allows owl users to add any attribute to the list of
translatable attributes.

closes #903

tests/qweb/__snapshots__/qweb.test.ts.snap

@@ -3943,6 +3943,20 @@ exports[`t-set value priority 1`] = `
 }"
 `;
 
+exports[`translation support can add additional attributes to the list of translatable attributes 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"test\\"
+    let h = this.h;
+    let _1 = 'word';
+    let _2 = 'mot';
+    let c3 = [], p3 = {key:3,attrs:{tomato: _1,potato: _2}};
+    let vn3 = h('div', p3, c3);
+    c3.push({text: \`text\`});
+    return vn3;
+}"
+`;
+
 exports[`translation support can translate node content 1`] = `
 "function anonymous(context, extra
 ) {

tests/qweb/qweb.test.ts

@@ -1,4 +1,5 @@
 import { QWeb } from "../../src/qweb/index";
+import { config } from "../../src/index";
 import { nextTick, normalize, renderToDOM, renderToString, trim } from "../helpers";
 import { patch } from "../../src/vdom";
 
@@ -13,6 +14,7 @@ let qweb: QWeb;
 
 beforeEach(() => {
   QWeb.TEMPLATES = {};
+  QWeb.nextId = 1;
   qweb = new QWeb();
 });
 
@@ -2211,6 +2213,17 @@ describe("translation support", () => {
     );
   });
 
+  test("can add additional attributes to the list of translatable attributes", () => {
+    const translations = {
+      word: "mot",
+    };
+    const translateFn = (expr) => translations[expr] || expr;
+    const qweb = new QWeb({ translateFn });
+    config.translatableAttributes.push("potato");
+    qweb.addTemplate("test", `<div tomato="word" potato="word">text</div>`);
+    expect(renderToString(qweb, "test")).toBe('<div tomato="word" potato="mot">text</div>');
+  });
+
   test("translation is done on the trimmed text, with extra spaces readded after", () => {
     const translations = {
       word: "mot",