Add filters for get_post_galleries() and get_post_gallery()

[alexkingorg]

As post format implementations were not tightly defined in the past, there are a variety of gallery implementations in the wild. For example, not all implementations included the use of the [gallery] shortcode.

Adding in a couple of hooks will likely allow for backward compatibility to those implementations.

[alexkingorg]

add filters to Gallery post format functions

[nacin]

As said in IRC:

• as long as passed args make sense, good for commit from me.
• but: it does make me ask why get_post_gallery() does not call get_post_galleries().
• if there is a filter in the latter, then the former really needs to wrap the latter.

Going to look at this in concert with #24202 and a general audit of these functions.

[alexkingorg]

:does a happy little dance:

[helen]

Also, that get_content_galleries() only takes 3 args.

[nacin]

In 24682:

Simplify and reduce the new media/content extraction functions.

The URL extraction function is now get_url_in_content(). For more, see #24202.

Also adds filters to get_post_galleries() and get_post_gallery(). fixes #24309.