Add a new Drupal.theme JavaScript function for theming the the show/hide row weight toggle and its wrapper in draggable tables

Problem/Motivation

Drupal core's tabledrag.js uses a hard-coded markup for the tabledrag toggle height markup and its wrapper div that are not overridable by themes.

Proposed resolution

1. Introduce new Drupal.theme callback for the toggle weight (visibility) button.
2. Introduce new Drupal.theme callback for the toggle button wrapper.
3. Refactor tabledrag.js and fully replace the toggle if it is changed. This is required for adding different icons for the 'show' and the 'hide' action.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet