[FIX] slots: fix bad interaction between t-call and slots

The previous changes in the combine method (used to copy all the
variables defined in the current scope for use in a slot) had the effect
of squashing the prototype chain: instead of `Component -> Obj1 ->
Obj2 -> Obj3`, the combined scope had: `Component -> Obj1'`.

This has an unfortunate interaction with the way t-call is implemented,
which uses the fact that we are in a subscope to add a own
__access_mode__. It depends specifially on the prototype chain, and that
the parent scope may have a different value for that property. But with
the way combine was implemented, we lost all that subtlety since
everything is squashed.

In this commit, we reimplement that function in a way to make sure we
keep the prototype chain structure

src/qweb/qweb.ts

@@ -137,12 +137,26 @@ const UTILS: Utils = {
     }
     return result;
   },
+  /**
+   * This method combines the current context with the variables defined in a
+   * scope for use in a slot.
+   * 
+   * The implementation is kind of tricky because we want to preserve the
+   * prototype chain structure of the cloned result. So we need to traverse the
+   * prototype chain, cloning each level respectively.
+   */
   combine(context, scope) {
-    const clone = Object.create(context);
+    let clone = context;
+    const scopeStack = [];
     while (!isComponent(scope)) {
-      Object.assign(clone, scope);
+      scopeStack.push(scope);
       scope = scope.__proto__;
     }
+    while (scopeStack.length) {
+      let scope = scopeStack.pop();
+      clone = Object.create(clone);
+      Object.assign(clone, scope);
+    }
     return clone;
   },
   shallowEqual,