Block - move to xml mappings

Can be merged after https://github.com/symfony-cmf/BlockBundle/pull/89 is merged and the dependencies for it are updated.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | na
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/199