[FIX] qweb: properly supports svg

Snabbdom supports svg, but it adds the namespace at the creation of the
virtual node.  However, owl works slightly differently: it adds children
after creating the parent virtual node.

So, we need to actually call the addNS method after the children nodes
have been created.

As a bonus, this is slightly faster than snabbdom: we only check at
template compilation time once if a node is a svg.

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

@@ -676,6 +676,31 @@ exports[`misc global 1`] = `
 }"
 `;
 
+exports[`properly support svg add proper namespace to svg 1`] = `
+"function anonymous(context,extra
+) {
+    let utils = this.constructor.utils;
+    var h = this.h;
+    var _1 = '100px';
+    var _2 = '90px';
+    let c3 = [], p3 = {key:3,attrs:{width: _1,height: _2}};
+    var vn3 = h('svg', p3, c3);
+    result = vn3;
+    var _4 = '50';
+    var _5 = '50';
+    var _6 = '4';
+    var _7 = 'green';
+    var _8 = '1';
+    var _9 = 'yellow';
+    let c10 = [], p10 = {key:10,attrs:{cx: _4,cy: _5,r: _6,stroke: _7,\\"stroke-width\\": _8,fill: _9}};
+    var vn10 = h('circle', p10, c10);
+    c3.push(vn10);
+    c3.push({text: \` \`});
+    utils.addNameSpace(vn3);
+    return vn3;
+}"
+`;
+
 exports[`special cases for some boolean html attributes/properties input type= checkbox, with t-att-checked 1`] = `
 "function anonymous(context,extra
 ) {

tests/qweb/qweb.test.ts

@@ -1312,3 +1312,10 @@ describe("global template registration", () => {
     expect((vnode as any).children[0].text).toBe("global");
   });
 });
+
+describe("properly support svg", () => {
+  test("add proper namespace to svg", () => {
+    qweb.addTemplate("test", `<svg width="100px" height="90px"><circle cx="50" cy="50" r="4" stroke="green" stroke-width="1" fill="yellow"/> </svg>`);
+    expect(renderToString(qweb, "test")).toBe(`<svg width=\"100px\" height=\"90px\"><circle cx=\"50\" cy=\"50\" r=\"4\" stroke=\"green\" stroke-width=\"1\" fill=\"yellow\"></circle> </svg>`);
+  });
+});