[IMP] qweb: add support for t-tag directive

Very useful in some cases, when one needs to define a generic component.

closes #721

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

@@ -0,0 +1,71 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`qweb t-tag simple usecases 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"test\\"
+    let scope = Object.create(context);
+    let result;
+    let h = this.h;
+    let c1 = [], p1 = {key:1};
+    let tag2 = 'div';
+    let vn1 = h(tag2, p1, c1);
+    result = vn1;
+    return result;
+}"
+`;
+
+exports[`qweb t-tag simple usecases 2`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"test\\"
+    let scope = Object.create(context);
+    let result;
+    let h = this.h;
+    let c3 = [], p3 = {key:3};
+    let tag4 = scope['tag'];
+    let vn3 = h(tag4, p3, c3);
+    result = vn3;
+    c3.push({text: \`text\`});
+    return result;
+}"
+`;
+
+exports[`qweb t-tag with multiple attributes 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"test\\"
+    let scope = Object.create(context);
+    let result;
+    let h = this.h;
+    let _2 = {'blueberry':true};
+    let _3 = 'raspberry';
+    let c4 = [], p4 = {key:4,attrs:{taste: _3},class:_2};
+    let tag5 = scope['tag'];
+    let vn4 = h(tag5, p4, c4);
+    result = vn4;
+    c4.push({text: \`gooseberry\`});
+    return result;
+}"
+`;
+
+exports[`qweb t-tag with multiple child nodes 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"test\\"
+    let scope = Object.create(context);
+    let result;
+    let h = this.h;
+    let c1 = [], p1 = {key:1};
+    let tag2 = scope['tag'];
+    let vn1 = h(tag2, p1, c1);
+    result = vn1;
+    c1.push({text: \` pear \`});
+    let c3 = [], p3 = {key:3};
+    let vn3 = h('span', p3, c3);
+    c1.push(vn3);
+    c3.push({text: \`apple\`});
+    c1.push({text: \` strawberry \`});
+    return result;
+}"
+`;