Add filter to page_template_dropdown()

[meloniq]

page_template_dropdown() is used in 'Page Attributes' metabox to list available page templates. The filter hook will give ability to adjust what's displayed in this box.

Our issue:
We use many custom templates to perform some specific tasks like building frontend dashboards, submit/edit forms, login pages, etc.
As far it works well, but the 'Page Attributes' metabox fulfill with page templates that in most of cases are not intended for any other use, and some administrators get confused seeing full list of available templates while creating a new page.

[meloniq]

Adds 'page_template_dropdown' filter

[SergeyBiryukov]

Appears to be a duplicate of #13265 and #21891.

[SergeyBiryukov]

The move to selected() looks good though, touch_time() and parent_dropdown() would benefit from that as well. I'd suggest creating a new ticket for that.

[meloniq]

Created separate ticket for the selected() use, and documenting this functions, see: #25889

Referenced tickets touches similar item (filtering page templates) but on lower level, using that filter to change items in dropdown could cause unexpected issues, as it will remove templates from all places, and not only from mentioned dropdown.

[meloniq]

Adds 'page_template_dropdown' filter (without selected() and docs changes)

[SergeyBiryukov]

My only concern with 'page_template_dropdown' filter name is that filters of the same name as the function are generally used to filter the actual result of that function. I'd suggest 'page_template_dropdown_items'.

[SergeyBiryukov]

The new filter would also need to be documented:
​http://make.wordpress.org/core/handbook/inline-documentation-standards/php-documentation-standards/#4-hooks-actions-and-filters

[meloniq]

Renamed filter to 'page_template_dropdown_items', added inline filter doc

[iamfriendly]

Refreshed the patch

[iamfriendly]

Refreshed the patch as the function has moved locations to template-functions.php and also updated the docs to reflect this would be for 4.4.0

[DrewAPicture]

In 34995:

Template: Make it possible to both add and remove items from the page templates list using the theme_page_templates filter.

The theme_page_templates hook was originally added in [27297] as page_templates, and later renamed in [27470]. Previously, it was only possible to remove or rename page templates via this hook.

Fixes #13265. Fixes #25879.