Improve Composite element and multiple element #states API support.

Problem

Composite element and multiple elements should fully support client-side and server-side conditional logic.

Solution

Improve Composite element and multiple element #states API support.

Tasks

• Setup test forms form composite and multiple elements.
• webform.webform.test_form_states_server_multiple.yml
• webform.webform.test_form_states_server_composite.yml
• Audit conditional behavior and better document concepts.
• Don't visually require the more items input element.
• Fix single composite element
• Fix multiple single element
• Fix multiple composite without required sub-elements
• Fix multiple composite with required sub-elements
• Check custom composite elements
• Write tests

Notes

• We might need to track and cache an element's #states API.