getUniqueIdentifier()
'''''''''''''''''''''

Returns a unique identifier of the element. While element identifiers are only unique within one form,
this identifier includes also the identifier of the form itself, and therefore making it "globally" unique.

Signature:

.. code-block:: php

    public function getUniqueIdentifier(): string;

Example:

.. code-block:: yaml

    identifier: exampleForm
    label: 'Simple Contact Form'
    prototype: standard
    type: Form

    renderables:
      -
        identifier: page-1
        label: 'Contact Form'
        type: Page

        renderables:
          -
            identifier: name
            label: 'Name'
            type: Text
            defaultValue: ''

.. code-block:: php

    // $formElement->getIdentifier() == 'name'
    $uniqueIdentifier = $formElement->getUniqueIdentifier();
    // $uniqueIdentifier == 'exampleForm-name'
