[4.0] Add optional xmlsource attribute to XML form fieldset - like formsource attribute of subform field type

[BrainforgeUK]

Steps to reproduce the issue

Instead of writing this:

[BrainforgeUK]

Instead of writing this:
<fieldset name="myfieldset1" >
<field name="myfield1" type="text" label="MYFIELD1" />
<!-- lots of other fields go here ->
</fieldset>

Pull-in the fields in the field set from another file
<fieldset name="myfieldset1" xmlsource="myfield1fields.xml" />

This optional attribute would work the same as the formsource attribute of the subform field type.

Additional comments

When a form has a number of fieldsets this will improve code creation, readbility and maintainability.

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