Descriptive names for renderable elements

Problem

We define several renderable elements in hook_element_info(). Some of them may look alike at first sight, such as the elements for displaying or configuring line items. We also already have an element for displaying payment statuses, and #1932430: Add a payment status update page will add an element for configuring one. The names tend to get confusing.

Proposal

Suffix every input element with _input and every display element with _display. Also use singular and plural where appropriate.