[REF] owl: move some exported values around

This is a breaking change. Code using Observer, EventBus, Store or
ConnectedComponent need to update the way they import theses classes.

- Observer is now in owl.misc (so you can import it like this:
    const Observer = owl.misc.Observer;)
- EventBus is also now in owl.misc
- Store and ConnectedComponent are now together in store

tools/playground/samples.js

@@ -338,7 +338,7 @@ function makeStore() {
         todos,
         nextId
     };
-    const store = new owl.Store({
+    const store = new owl.store.Store({
         state,
         actions,
         mutations
@@ -405,7 +405,7 @@ class TodoItem extends owl.Component {
 //------------------------------------------------------------------------------
 // TodoApp
 //------------------------------------------------------------------------------
-class TodoApp extends owl.ConnectedComponent {
+class TodoApp extends owl.store.ConnectedComponent {
     components = { TodoItem };
     state = { filter: "all" };