fix form alias

The dot `.` is not allowed in the form alias (or form type name), so the dot should be replaced by underscore in the compiler pass, otherwise the form type can't be found (in a situation that the form type is created manually and the entity is not in the same bundle as the form type). Of course, to make it work, the form type's `getName` method should return the proper alias name (eg. `app_form_new_task_type`)

I also suggest that the bundle name `app` should not hard coded in the form alias when `FormTypeCreator` try to find the proper form type with entity name, currently the form alias format is 

```php
sprintf('app_form_%s%s_type', $purpose, $shortEntityClassName);
```

it could be better if change it to:

```php
sprintf('%s_%s%s', $entityBundleName, $purpose, $shortEntityClassName);
```

The `form` and `type` are also be removed because they seem pointless and tedious(it must be a FORM TYPE's alias)

I'll update this PR if you agree with that. :)