[FIX] slots: make sure handlers are properly bound to component

In some situations (a slot inside a slot), the combine method was
wrongly copying all properties of the scope into the context, which
caused the event handling system to wrongly use a subobject as component
(since it detects the fact that __owl__ is a own property_).
Consequently, we could have very subtle issue with some properties being
shadowed by a sub object.

src/qweb/qweb.ts

@@ -139,8 +139,9 @@ const UTILS: Utils = {
   },
   combine(context, scope) {
     const clone = Object.create(context);
-    for (let k in scope) {
-      clone[k] = scope[k];
+    while (!isComponent(scope)) {
+      Object.assign(clone, scope);
+      scope = scope.__proto__;
     }
     return clone;
   },