imp: qweb compiler now condenses whitespaces

This is a pretty big breaking change: the html output by qweb is now
different.

The main goal is to optimize the compiled templates:
- condense all consecutive whitespaces into a single spaces
- if possible, drop completely some text nodes, based on the following
  heuristic: if a text node is only composed of whitespaces, and
contains at least one linebreak, then it can be dropped.

This leads in my benchmark test to an improvement of about 10%, in
rendering speed and in memory consuption.

Note: whitespace here means anything that matches the \s regexp: newlines,
tabs, ...

close #8

examples/todoapp/components/TodoApp.js

@@ -33,7 +33,8 @@ class TodoApp extends Component {
   }
 
   get remainingText() {
-    return (this.remaining < 2 ? "item" : "items") + " left";
+    const items = this.remaining < 2 ? "item" : "items";
+    return ` ${items} left`;
   }
 
   addTodo(ev) {