Fix for: Field subform (multiple) produces wrong id

[Fedik]

Pull Request for Issue #15187 .

Summary of Changes

The pull fix the way how the script calculate the id for a fields and it's labels.
Also now it should be more close to JFormField::getId result.

Testing Instructions

For test can use next XML (place it in some params form,eg in template or custom module):

<field type="subform" name="subform" label="Subform" multiple="true">
    <form>
        <field type="radio" name="radio1" label="Radio">
            <option value="1">Val 1</option>
            <option value="2">Val 2</option>
        </field>
        <field type="checkboxes" name="checkboxes1" label="checkboxes">
            <option value="3">Val 3</option>
            <option value="4">Val 4</option>
        </field>
        <field type="checkbox" name="checkbox1" label="checkbox" value="1"/>
        <field type="text" name="text1" label="text1"/>
    </form>
</field>

Expected result

After add "new row", check the input/labels id, they should be similar to screenshot:

Actual result

ID of the labels is not fixed,
they contain X eg: ..._subform__subformX_..

Documentation Changes Required

none

[degobbis]

I have tested this item ✅ successfully on bf44558
_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/16480.}

[drmenzelit]

I have tested this item ✅ successfully on bf44558

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/16480.}

[ghost]

RTC after two successful tests.