Fixed dot notation of nested fields in forms.

The dot notation of nested fields in forms `$formMapper->add('user.surname');` was available in Sonata 2.0, but as far as I know, in next versions it was broken and ends with error: 

    The name "user.surname" contains illegal characters. Names should start with a letter, digit or underscore and only contain letters, digits, numbers, underscores ("_"), hyphens ("-") and colons (":").',`

The reason was that Form Component does not allow dot in form name anymore and we should change the `name`, set the `type` and `property_path` option, too. 

This PR fix this problem and reintroduce the "dot" notation feature in forms.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #919 (filter forms was fixed before, this fix is for standard form)
| License       | MIT
| Doc PR        | -