[IMP] config: add enableTransitions flag to config

It is useful to be able to disable the t-transition directive for
testing purposes.  This commit introduces a global config flag to do
just that.

closes #768

src/config.ts

@@ -8,6 +8,7 @@ import { QWeb } from "./qweb/index";
 
 interface Config {
   mode: string;
+  enableTransitions: boolean;
 }
 
 export const config = {} as Config;
@@ -28,3 +29,12 @@ Object.defineProperty(config, "mode", {
     }
   },
 });
+
+Object.defineProperty(config, "enableTransitions", {
+  get() {
+    return QWeb.enableTransitions;
+  },
+  set(value: boolean) {
+    QWeb.enableTransitions = value;
+  },
+});

src/qweb/extensions.ts

@@ -205,6 +205,9 @@ QWeb.addDirective({
   name: "transition",
   priority: 96,
   atNodeCreation({ ctx, value, addNodeHook }) {
+    if (!QWeb.enableTransitions) {
+      return;
+    }
     ctx.rootContext.shouldDefineUtils = true;
     let name = value;
     const hooks = {

src/qweb/qweb.ts

@@ -212,6 +212,7 @@ export class QWeb extends EventBus {
   h = h;
   // dev mode enables better error messages or more costly validations
   static dev: boolean = false;
+  static enableTransitions: boolean = true;
 
   // slots contains sub templates defined with t-set inside t-component nodes, and
   // are meant to be used by the t-slot directive.