[FIX] slots: better heuristic to determine named slot content

Unfortunately, we chose to use the directive `t-set` to define sub slot
contents in a template.

The goal was to reuse a directive for a similar use case: defining sub
template is almost the same as defining a slot content.

Obviously, this introduces a name conflict: the inner content of a
component cannot use t-set t-value anymore (nor t-set with a body
value), since they are interpreted as slot names.

We mitigate the issue here by only interpreting as slot content the
`t-set` statement located immediately below the parent component tag
name and with a body content.

However, a real fix need to introduce an additional directive to resolve
the ambiguity.

src/component/directive.ts

@@ -400,7 +400,12 @@ QWeb.addDirective({
 
     if (hasSlots) {
       const clone = <Element>node.cloneNode(true);
-      const slotNodes = clone.querySelectorAll("[t-set]");
+      const slotNodes: Element[] = [];
+      for (let el of clone.children) {
+        if (el.getAttribute("t-set") && el.hasChildNodes()) {
+          slotNodes.push(el);
+        }
+      }
       const slotId = QWeb.nextSlotId++;
       ctx.addLine(`w${componentID}.__owl__.slotId = ${slotId};`);
       if (slotNodes.length) {