QfqForm.js: support `required` configuration attribute.

Support for `data-hidden` and `data-disabled` attributes, which are used to initially configure form elements.

file.php: API described.
Store.php: Fixed STORE_EXTRA initialization bug
AbstractBuildForm.php: code rearranged.
Constants.php:  UPLOAD added

Preview commit for field validation using bootstrap-validator. Do a `make boostrap && grunt` after merge

QfqForm.js: Buttons should behave as described in CODING.md under UI.

Form.js: Textarea also get input and paste event listener in order to fire upon changes immediately.

Fixed bug in QfqForm#handleNewClick(): Use event.deledateTarget instead of event.target. From jQuery Documentation:

event.delegateTarget::
 The element where the currently-called jQuery event handler was attached.

That's what we want, because we require the href attribute of anchor.