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>
````

## Todo

 - [x] Create the `.xsd` file
 - [x] Add namespace (`http://cmf.symfony.com/schema/dic/block`)