Customize: Remove call to `wp_trash_post()`

[dlh]

WP_Customize_Manager::handle_changeset_trash_request() includes a call to wp_trash_post(), which was purposefully not called in _wp_customize_publish_changeset() because it "will mutate the post_content and the post_name."

As discussed in Slack, the logic for trashing a changeset could be moved to a method in WP_Customize_Manager that both functions can use: ​https://wordpress.slack.com/archives/C0381N237/p1507503461000055.

Note that it might make sense for this logic to one day be moved again to a dedicated WP_Customize_Changeset class. The method in the attached patch is largely borrowed from the latest patch in #40527.

The patch also adds the 'pre_trash_post' filter from [41638].

[westonruter]

In 41824:

Customize: Introduce WP_Customize_Manager::trash_changeset_post() to reduce duplication and ensure proper changeset trashing logic.

Trashing a changeset via wp_trash_post() does not have the desired result since it mutates post_content (via Kses) and the post_name (even though it is a UUID).

Props dlh.
See #39896, #42030.
Fixes #42175.