[FIX] qweb, t-call: t-call nested in t-slot

Have a t-call within a t-set-slot of a component.
The called template has a t-component directive.

It should be like:

```xml
<t t-name="Zero">
  <Slotted>
    <t t-call="someTemplate" />
  </Slotted>
</t>

<t t-name="someTemplate">
  <SomeComponent />
</t>
```

Before this commit, the parent of SomeComponent was the Zero component

After this commit, the parent of SomeComponent is the Slotted Component as it should be

closes #862

src/qweb/base_directives.ts

@@ -281,7 +281,7 @@ QWeb.addDirective({
 
     // Step 4: add the appropriate function call to current component
     // ------------------------------------------------
-    const parentComponent = `utils.getComponent(context)`;
+    const parentComponent = ctx.rootContext.shouldDefineParent ? `parent` : `utils.getComponent(context)`;
     const key = ctx.generateTemplateKey();
     const parentNode = ctx.parentNode ? `c${ctx.parentNode}` : "result";
     const extra = `Object.assign({}, extra, {parentNode: ${parentNode}, parent: ${parentComponent}, key: ${key}})`;

src/qweb/qweb.ts

@@ -436,6 +436,7 @@ export class QWeb extends EventBus {
       ctx.variables = Object.create(null);
       ctx.parentNode = ctx.generateID();
       ctx.allowMultipleRoots = true;
+      ctx.shouldDefineParent = true;
       ctx.hasParentWidget = true;
       ctx.shouldDefineResult = false;
       ctx.addLine(`let c${ctx.parentNode} = extra.parentNode;`);