Field widget sub-field sorting [#3585300]

Problem/Motivation

The sub-fields sorting is driven off of the form display settings but this isn't respected in the default values input in the field settings form. The logic for sorting can also be simplified in a single function in the CustomWidgetBase and thus we can reduce code in the widgets that extend it.

Proposed resolution

In widget base, Get rid of sortFields() and refactor the getCustomFieldItems() to include sorting and current state so it can be reused.

Issue fork custom_field-3585300

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3585300-field-widget-sub-field changes, plain diff MR !218
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