[FIX] qweb: allow t-call on arbitrary html nodes

This is a rarely (if ever) used feature, but according to our qweb
reference implementation, it is possible to use the
t-call directive on an arbitrary html tag, like this:

<div t-call="my.template"/>

It is then interpreted as:

<div><t t-call="my.template"/></div>

So, with this commit, we make sure that the owl qweb implementation
matches that behaviour.

closes #706

src/qweb/base_directives.ts

@@ -223,9 +223,6 @@ QWeb.addDirective({
     // ------------------------------------------------
     ctx.rootContext.shouldDefineScope = true;
     ctx.rootContext.shouldDefineUtils = true;
-    if (node.nodeName !== "t") {
-      throw new Error("Invalid tag for t-call directive (should be 't')");
-    }
     const subTemplate = node.getAttribute("t-call")!;
     const nodeTemplate = qweb.templates[subTemplate];
     if (!nodeTemplate) {

src/qweb/qweb.ts

@@ -517,10 +517,17 @@ export class QWeb extends EventBus {
       return;
     }
 
+    if (node.tagName !== "t" && node.hasAttribute("t-call")) {
+      const tCallNode = document.createElement("t");
+      tCallNode.setAttribute("t-call", node.getAttribute("t-call")!);
+      node.removeAttribute("t-call");
+      node.prepend(tCallNode);
+    }
+
     const firstLetter = node.tagName[0];
     if (firstLetter === firstLetter.toUpperCase()) {
       // this is a component, we modify in place the xml document to change
-      // <SomeComponent ... /> to <t t-component="SomeComponent" ... />
+      // <SomeComponent ... /> to <SomeComponent t-component="SomeComponent" ... />
       node.setAttribute("t-component", node.tagName);
     } else if (node.tagName !== "t" && node.hasAttribute("t-component")) {
       throw new Error(