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.

Removed global EventEmitter. Added local EventEmitter to Object really needing it. Alert.js now uses EventEmitter.

 * use anonymous functions for ajax error displays
 * expect response from server to file upload to have `status` and `message` attributes
 * replaced occurrences of `QfqNS` by `n` in order to support different namespace names.

Do not address namespace with `QfqNS` in Object definitions. Instead, use `n`. This might have screwed up the code. We'll fix as we go along.

qfq-bs.css.less: Tweaked less. Added new class for spinner. Changed display type of input[type=file].
FileUpload.js: every callback will be called with the originating file input element as first parameter.
QfqForm.js: Add spinner on upload.