[WIP] Refactor config to better support XML

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | tbd

XML support was pretty buggy. This PR introduces out-of-the-box support for XML by using the same technique as the Validator component:

````xml
<content-path name="pages">
    <provider>
        <option name="name" value="specified" />
        <option name="path" value="/cms/routing/page" />
    </provider>

    <exists-action>
        <option name="name" value="strategy" />
        <option name="value" value="use" />
    </exists-action>

    <not-exists-action>
        <option name="name" value="strategy" />
        <option name="value" value="create" />
    </not-exists-action>
</content-path>
````

The Yaml and PHP formats keep the same.

This PR also tests the configuration class by using @matthiasnoback's SymfonyConfigTest component. This needs to be integrated into the Testing component sometime.