[Forms] add missing features being new in 2.3

These features are:

* Use the submit() (instead of bind()) method to manually submit a
  form.
* The ability to change a form's action and method.
feature #4085 [Component][Forms] add missing features introduced in 2.3 (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[Component][Forms] add missing features introduced in 2.3

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#6522)
| Applies to    | all
| Fixed tickets | #2969

- [x] use ``submit()`` instead of ``bind()`` to manually submit a form
- [x] configure method and action of a form
- [ ] ~~render a form's start and its end separately~~ (see [the comment](https://github.com/symfony/symfony-docs/pull/4085#issuecomment-52386947))
- [ ] configure buttons

Commits
-------

33f6422 [Forms] add missing features being new in 2.3