Add page template filter for all post types

[desrosj]

In 4.7, a dynamic filter was introduced that filters page templates. It would be great if there were a filter for all post types as well.

Use case: I have a page template in a theme that uses a class in a plugin. If the plugin is not present on the site, that page template should be filtered out of the page template list. Right now, you have to register a filter for every post type.

Related: #18375

[desrosj]

43872.diff​ introduces a theme_templates filter for filtering page templates regardless of post type.

[SergeyBiryukov]

In 43025:

Themes: Introduce theme_templates filter for page templates of all post types.

This complements the theme_{$post_type}_templates dynamic filter added in [38951].

Props desrosj.
Fixes #43872.

[SergeyBiryukov]

Looks like we have a precedent of adding new hooks in recent minor releases, see [42589] for example.

This seems like a good enhancement for 4.9.6.

[SergeyBiryukov]

In 43027:

Docs: Update @since version for theme_templates filter added in [43025].

See #43872.

[SergeyBiryukov]

In 43028:

Themes: Introduce theme_templates filter for page templates of all post types.

This complements the theme_{$post_type}_templates dynamic filter added in [38951].

Props desrosj.
Merges [43025] and [43027] to the 4.9 branch.
Fixes #43872.