[FIX] qweb/component: refactoring of scoping/variables/slots

This commit is a significant refactoring of the internal of QWeb. It
simplifies the way variables/scoping and slots interact together.  The
main idea is that we use a simple scope object instead of a
context/var/scope object.

This commit also implement the actual correct QWeb semantic for the
t-call directive with a sub body.  Before, we simply extracted the
variables from the body and injected them at the top of the sub
template.  We now simply compile the body before the sub template.

This is a joint work with Lucas (lpe).

closes #541
closes #544
closes #545

closes #557
closes #556

src/component/component.ts

@@ -70,10 +70,9 @@ interface Internal<T extends Env, Props> {
   parentLastFiberId: number;
 
   // when a rendering is initiated by a parent, it may set variables in 'scope'
-  // and 'vars' (typically when the component is rendered in a slot). We need to
+  // (typically when the component is rendered in a slot). We need to
   // store that information in case the component would be re-rendered later on.
   scope: any;
-  vars: any;
 
   boundHandlers: { [key: number]: any };
   observer: Observer | null;
@@ -198,8 +197,7 @@ export class Component<T extends Env, Props extends {}> {
       renderFn: qweb.render.bind(qweb, template),
       classObj: null,
       refs: null,
-      scope: null,
-      vars: null
+      scope: null
     };
   }
 
@@ -510,9 +508,8 @@ export class Component<T extends Env, Props extends {}> {
    * The __updateProps method is called by the t-component directive whenever
    * it updates a component (so, when the parent template is rerendered).
    */
-  async __updateProps(nextProps: Props, parentFiber: Fiber, scope: any, vars: any): Promise<void> {
+  async __updateProps(nextProps: Props, parentFiber: Fiber, scope: any): Promise<void> {
     this.__owl__.scope = scope;
-    this.__owl__.vars = vars;
     const shouldUpdate = parentFiber.force || this.shouldUpdate(nextProps);
     if (shouldUpdate) {
       const __owl__ = this.__owl__;
@@ -566,12 +563,11 @@ export class Component<T extends Env, Props extends {}> {
 
   /**
    * The __prepare method is only called by the t-component directive, when a
-   * subcomponent is created. It gets its scope and vars, if any, from the
+   * subcomponent is created. It gets its scope, if any, from the
    * parent template.
    */
-  __prepare(parentFiber: Fiber, scope: any, vars: any, cb: CallableFunction): Fiber {
+  __prepare(parentFiber: Fiber, scope: any, cb: CallableFunction): Fiber {
     this.__owl__.scope = scope;
-    this.__owl__.vars = vars;
     const fiber = new Fiber(parentFiber, this, parentFiber.force, null);
     fiber.shouldPatch = false;
     if (!parentFiber.child) {