Add support for inspecting theme blocks

[bakura10]

As of today, if a section is using theme blocks, the only thing we can do is rendering them. However, it is not possible to access the settings of theme blocks nor their types:

{% for block in section.blocks %}
  {{ block.type }} => always return @theme
  {{ block.settings.foo }} => returns nothing
{% endfor %}

This is problematic for complex sections. For instance, we have sections that must have some CSS properties defined based on a max value of children blocks. There are cases where we don't really care about the order, but more about the content itself.

The current limitation where theme blocks can only be rendered with {% content_for %} is way too limited. We should be able to inspect them and render a block in unit:

{% for block in section.blocks %}
  {% if block.type == '@theme/foo' %}
    {% render block %}
  {% endif %}
{% endfor %}