[FIX] qweb: correctly handle top level t-call and body

Whenever a top level t-call was made with some non empty body, Owl
complained that a template should not have more than one root node.

The reason was that the compilation context for the body of the t-call
directive was the same as the root compilation context, and it already
had a parentNode set.

To fix the issue, this commit simply use the subContext method to create
a different compilation context, which actually makes sense, because the
body of a t-call is really a different situation.  Also, as a bonus, it
slightly improves the code for the t-call directive.

closes #760

src/qweb/base_directives.ts

@@ -267,16 +267,13 @@ QWeb.addDirective({
       for (let attr of ["t-if", "t-else", "t-elif", "t-call"]) {
         nodeCopy.removeAttribute(attr);
       }
-      const parentNode = ctx.parentNode;
-      ctx.parentNode = "__0";
       // this local scope is intended to trap c__0
       ctx.addLine(`{`);
       ctx.indent();
       ctx.addLine("let c__0 = [];");
-      qweb._compileNode(nodeCopy, ctx);
+      qweb._compileNode(nodeCopy, ctx.subContext("parentNode", "__0"));
       ctx.rootContext.shouldDefineUtils = true;
       ctx.addLine("scope[utils.zero] = c__0;");
-      ctx.parentNode = parentNode;
       ctx.dedent();
       ctx.addLine(`}`);
     }