[For 2.6] Add drag and drop API to list

[dweymouth]

Description:

Adds a drag-and-drop API to List for reordering items. This is the implementation from github.com/dweymouth/fyne-advanced-list ported over for review and hopeful inclusion in Fyne 2.6

Fixes #(issue)

Checklist:

• [ ] Tests included.
• [ ] Lint and formatter run with no errors.
• [x] Tests all pass.

Where applicable:

• [ ] Public APIs match existing style and have Since: line.
• [ ] Any breaking changes have a deprecation path or have been discussed.
• [ ] Check for binary size increases when importing new modules.