[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

doc/component.md

@@ -146,11 +146,6 @@ find a template with the component name (or one of its ancestor).
   if we have a `isMobile` key in the environment, to decide if we want a mobile
   interface or a destkop one.
 
-- **`set(target, key, value)`**. This method is necessary in some cases when we
-  need to modify the state of the component in a way that is not visible to the
-  observer (see [observer's technical limitations](observer.md#technical-limitations)).
-  For example, if we need to add a key to the state.
-
 - **`destroy()`**. As its name suggests, this method will remove the component,
   and perform all necessary cleanup, such as unmounting the component, its children,
   removing the parent/children relationship. This method should almost never be

doc/observer.md

@@ -20,35 +20,31 @@ obj.a.b = 2;
 ## Technical Limitations
 
 Since the observer uses getters and setters, it is actually unable to react to
-changes in two situations:
+changes in three situations:
 
-- adding a key to an object:
+- adding a key to an object
+- deleting a key from an object
+- modifying an array by setting a new value at a given index
+
+In those situations, we need a way to tell the observer that something happened.
+This can be done by using the `set` and `delete` (only for objects) static
+methods of the `Observer`.
 
   ```javascript
   const observer = new owl.Observer();
   const obj = { a: 1 };
   observer.observe(obj);
-  obj.b = 2; // will do nothing
+  obj.b = 2; // won't notify the change
+  owl.Observer.set(obj, "b", 2); // will notify the change
+
+  delete obj.b; // won't notify the change
+  owl.Observer.delete(obj, "b"); // will notify the change
   ```
 
-  In that case, we need a way to tell the observer that something happened.
-  This can be done by using the `set` method:
-
-  ```javascript
-  observer.set(obj, "b", 2);
-  ```
-
-- modifying an array by setting a new value at a given index:
-
   ```javascript
   const observer = new owl.Observer();
-  const obj = { todos: [{ id: 1, text: "todo" }] };
-  observer.observe(obj);
-  obj[0] = { id: 2, text: "othertodo" }; // will do nothing, and obj[0] is not observed
-  ```
-
-  In that case, the solution is the same, we can simply use the `set` method:
-
-  ```javascript
-  observer.set(obj, 0, { id: 2, text: "othertodo" });
+  const arr = ["a"];
+  observer.observe(arr);
+  arr[0] = "b"; // won't notify the change
+  owl.Observer.set(arr, 0, "b"); // will notify the change
   ```