Added callback with possibility to modify each individual item in res…

[klavig]

Subject

I am targeting this branch, because this PR adds extra functionality to sonata_type_model_autocomplete, fixes few glitches & extends customization options.

Changelog

### Added
- A callback with a possibility to modify each individual item in the response.
- A way to customize any autocomplete select2 properties in js via adding sonata_type_model_autocomplete_select2_options_js block before initializing select2 element because select2 doesn't allow modification of several options after it has been initialized, you can only destroy it and initialize again from scratch.
- Set error bubbling to false - prevent passing error to parent form elements, which occurs by default for compound form types.

### Changed
- The autocomplete item list is now independent of filters set in the corresponding list via removing its filters. Before these changes, it returned only items that matched the selected filter.

[klavig]

@VincentLanglet hey, I have re-created the PR, maybe u got the time to look at it

[OskarStark]

Can you please add this to the docs, otherwise this feature could not be adopted well, thanks!

[VincentLanglet]

Here: https://github.com/sonata-project/SonataAdminBundle/blob/29bbbb0da7270e112546e043021211bebdab9fff/docs/reference/form_types.rst

[klavig]

@VincentLanglet, @OskarStark done