Removing an item from a multiple value field widget discards the weight order

Problem/Motivation

Widgets for fields with unlimited cardinality show a "Remove" button for each row. When a row is removed, weights are recalculated in WidgetBase::deleteSubmit(). The code ignores the submitted weights and assigns new ones in the order the items were submitted, which is the order of the rows in the DOM.

When items are reordered via drag and drop, tabledrag moves the table rows in the DOM, so they are submitted in the chosen order and the bug is hidden.
When "Show row weights" is used (or JavaScript is disabled), the rows do not move in the DOM and the order is only indicated by the weight selects. Removing an item will discard the weights and the widget will show the remaining items in the original order.

Steps to reproduce

1. Create an unlimited-cardinality field (e.g. a plain text field) on a content type and go to the node add form.
2. Enter values 1, 2, 3 in three items.
3. Click "Show row weights" on the widget table.
4. Using the weight selects, set the weights to 2, 0, -2 so the intended order is 3, 2, 1.
5. Press the "Remove" button of the item containing 2.

Expected: the rebuilt widget shows 3, 1.
Actual: the rebuilt widget shows 1, 3.

Proposed resolution

In WidgetBase::deleteSubmit(), sort the remaining items by their submitted _weight before renumbering the weights.

Remaining tasks

Review.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

Removing an item from a multiple value field widget no longer discards the item order set through the row weight selects.