[6.1] Refactor subform rendering with fieldsets support

[BrainforgeUK]

groupByFieldset functionality added.

Summary of Changes

Setting groupByFieldset parameter in XML to true does not currently serve any purpose.

This change (without any other fieldset parameters) uses CSS grid display to group fields in a fieldset.

Additional parameters can be used with the fieldset(s) can be used to customise how the fields are grouped (e.g. use flex instead of grid).

Testing Instructions

Install the following plugins:
bfsubfieldtest1.zip
bfsubfieldtest2.zip
bfsubfieldtest3.zip

Actual result BEFORE applying this Pull Request

Go to plugin admin page.
Unexpected result - there is no difference between bfsubfieldtest1 and bfsubfieldtest2.

Expected result AFTER applying this Pull Request

Go to plugin admin page.
With bfsubfieldtest2 the the fields are now grouped by fieldset.
bfsubfieldtest3 illustrates how the appearance of fields grouped within fieldsets can be customised.

Link to documentations

The code in this pull request includes comments about the available fieldset attributes. These comments together with the example subform fields from bfsubfieldtest3 need to be included in the documentation.
Please select:

• [*] Documentation link for docs.joomla.org:
  Avanced Form Guide
  Subform Form Field

• [] No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[ceford]

I don't quite understand the testing instructions! The diff shows changes to layouts/joomla/form/field/subform/default.php - perhaps a screenshot would help.

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