Add support for nested blocks in templates

Nesting blocks would make the templates much more flexible. This way you can use more blocks and overwrite smaller parts of a base template.

[Twig also supports it](http://twig.sensiolabs.org/doc/templates.html#template-inheritance).

I also added more blocks to the `fe_page` templates to show how this could be used.

Related issue: #6508

### Example:

Base template: 

```html+php
<body>
  <?php $this->block('body'); ?>
    <div id="content">
      <?php $this->block('content'); ?>
        <p>I am the default content.</p>
      <?php $this->endblock(); ?>
    </div>
  <?php $this->endblock(); ?>
</body>
```

Extended template:

```html+php
<?php $this->extend('...'); ?>
<?php $this->block('body'); ?>
  <p>Extending body.</p>
  <?php $this->parent(); ?>
<?php $this->endblock(); ?>
<?php $this->block('content'); ?>
  <p>Replacing content.</p>
<?php $this->endblock(); ?>
```

Output:
```html
<body>
  <p>Extending body.</p>
  <div id="content">
    <p>Replacing content.</p>
  </div>
</body>
```