Add ux collection type component

[alexander-schranz]

Q	A
Bug fix?	no
New feature?	yes
Tickets	Fix #...
License	MIT

This PR is based on the awesome work of @stakovicz in: #90

TODO

• Move logic for data attribute from twig files to form types
• Use Symfony Button FormType for Add and Remove
• Implement Toolbar FormType for extendability over Form Extensions (Copy & Paste, ..)
• Add Toolbar and ButtonType over symfony Form for easier theming and extending
• Make JS component compatible with new form type and simplify the logic
• Fix possibility of overwrite data-controller for customization
• Keep nested collections in mind
• Find a way to use entry_type instead of ux_entry_type

[alexander-schranz]

The UXCollectionEntryToolbarType and UXCollectionToolbarType allows to add additional buttons later via a form extension. Example can be a Copy & Paste Functionality.

[dunglas]

Does this approach support nested dynamic collections (here is a test case: https://github.com/symfony/ux/tree/e901026aa1d2e2cd2b27a5796962bc8fc4b5dcca/src/Collection/Tests/app/Form)?

For the record, I proposed an alternative patch in #398 which doesn't require changing the existing forms to use this feature (see #398 (comment) for the rationale).

[alexander-schranz]

@dunglas yes you can have and I'm using nested collections this way. But like documented in the CollectionType you should use the different prototype_name to avoid conflicts between the 2 prototypes.