[IMP] observer: add set and delete static functions

This rev. add a 'set' and 'delete' static functions to the Observer.
They allow to set or delete (only for objects) properties on
observed objects or arrays.

With this, the 'set' functions in Store and Component are no longer
necessary, so they have been removed.

Closes #138

src/observer.ts

@@ -76,6 +76,24 @@ export class Observer {
   allowMutations: boolean = true;
   dirty: boolean = false;
 
+  static set(target: any, key: number | string, value: any) {
+    if (!target.__owl__) {
+      throw Error(
+        "`Observer.set()` can only be called with observed Objects or Arrays"
+      );
+    }
+    target.__owl__.observer.set(target, key, value);
+  }
+
+  static delete(target: any, key: number | string) {
+    if (!target.__owl__) {
+      throw Error(
+        "`Observer.delete()` can only be called with observed Objects"
+      );
+    }
+    target.__owl__.observer.delete(target, key);
+  }
+
   notifyCB() {}
   notifyChange() {
     this.dirty = true;
@@ -120,8 +138,19 @@ export class Observer {
     this.notifyChange();
   }
 
+  delete(target: any, key: number | string) {
+    delete target[key];
+    this._updateRevNumber(target);
+    this.notifyChange();
+  }
+
   _observeObj<T extends { __owl__?: any }>(obj: T, parent?: any) {
-    obj.__owl__ = { rev: this.rev, deepRev: this.rev, parent };
+    obj.__owl__ = {
+      rev: this.rev,
+      deepRev: this.rev,
+      parent,
+      observer: this
+    };
     Object.defineProperty(obj, "__owl__", { enumerable: false });
     for (let key in obj) {
       this._addProp(obj, key, obj[key]);
@@ -129,7 +158,12 @@ export class Observer {
   }
 
   _observeArr(arr: Array<any>, parent?: any) {
-    (<any>arr).__owl__ = { rev: this.rev, deepRev: this.rev, parent };
+    (<any>arr).__owl__ = {
+      rev: this.rev,
+      deepRev: this.rev,
+      parent,
+      observer: this
+    };
     Object.defineProperty(arr, "__owl__", { enumerable: false });
     (<any>arr).__proto__ = Object.create(ModifiedArrayProto);
     (<any>arr).__proto__.__observer__ = this;
@@ -145,6 +179,7 @@ export class Observer {
   ) {
     var self = this;
     Object.defineProperty(obj, key, {
+      configurable: true,
       enumerable: true,
       get() {
         return value;