…sing of type instances (webmozart)

This PR was merged into the 2.8 branch.

Discussion
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[Form] Deprecated FormTypeInterface::getName() and passing of type instances

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #5321, #15008
| License       | MIT
| Doc PR        | TODO

#### Type Names

This PR deprecates the definition of the `getName()` method of form types. See #15008 for a more detailed description.

Before:

```php
class MyType extends AbstractType
{
    public function getName()
    {
        return 'mytype';
    }

    // ...
}
```

After:

```php
class MyType extends AbstractType
{
    // ...
}
```

You should always reference other types by their fully-qualified class names. Thanks to PHP 5.5, that's easy:

Before:

```php
$form = $this->createFormBuilder()
    ->add('name', 'text')
    ->add('age', 'integer')
    ->getForm();
```

After:

```php
$form = $this->createFormBuilder()
    ->add('name', TextType::class)
    ->add('age', IntegerType::class)
    ->getForm();
```

#### Type Instances

Furthermore, passing of type instances is deprecated.

Before:

```php
$form = $this->createForm(new AuthorType());
```

After:

```php
$form = $this->createForm(AuthorType::class);
```

#### DIC Aliases

When registering a type in the DIC, you should omit the "alias" attribute now.

Before:

```xml
<service id="my.type" class="Vendor\Type\MyType">
    <tag name="form.type" alias="mytype" />
    <argument type="service" id="some.service.id" />
</service>
```

After:

```xml
<service id="my.type" class="Vendor\Type\MyType">
    <tag name="form.type" />
    <argument type="service" id="some.service.id" />
</service>
```

Types without dependencies don't need to be registered in the DIC as they can be instantiated right away.

#### Template Block Prefixes

By default, the class name of the type in underscore notation minus "Type" suffix is used as Twig template block prefix (e.g. `UserProfileType` => `user_profile_*`). If you want to customize that, overwrite the new `getBlockPrefix()` method in your type:

```php
class UserProfileType extends AbstractType
{
    public function getBlockPrefix()
    {
        return 'profile';
    }

    // ...
}
```

Commits
-------

3d9e5de [Form] Deprecated FormTypeInterface::getName() and passing of type instances