Improve XML configuration files

As discussed on the docs repo, I'll be doing some PRs to improved the XML configuration files.

This PR adds some `fixXmlConfig` methods to improve arrays in XML:

Before:
````xml
<multilang>
    <locales>en</locales>
    <locales>fr</locales>
</multilang>
````
After:
````xml
<multilang>
    <locale>en</locale>
    <locale>fr</locale>
</multilang>
````

We also need to think of a XML namespace. I thought about something like `http://cmf.symfony.com/schema/dic/block` (where `block` gets replaced with the bundle name for other bundles).

At last, we should create some `.xsd` files.