Add separate toolbar class to prevent style clash

The problem is that if you use `editor.css` as it is you are expected to have `<body class="g-class">` to make a toolbar work properly because toolbar styling relates on this fact.

Now you can have something like that:

```
<div class="g-class">
  <article contenteditable="true"></article>
</div>
```

Toolbar will be appended not inside `document.body` but inside special `<div class="g-body">` placed in the bottom of the `document.body`. This assures that any external CSS won't break the toolbar.