Allow retrieving adjacent image link markup without echoing it

[flixos90]

Currently, themes can only echo image links (via next_image_link(), previous_image_link() and indirectly adjacent_image_link()), but not retrieve the markup before printing it. This is a problem, since you often want to manually print extra markup, however only if there are actual image links to display. You can currently of course accomplish that by using an output buffer, but that is far from preferred.

Most other templating functions have two versions, one for retrieving the markup, the other for echoing it (for example previous_post_link() and get_previous_post_link()). We should add the necessary counterparts for these ones too.

As another indicator that this has been needed for a long time, you might want to look at this article from 2009: ​https://katz.co/911/

[flixos90]

45708.diff​ introduces the following functions:

• get_adjacent_image_link()
• get_next_image_link()
• get_previous_image_link()

Their whole code is what was previously part of the respective function without the get_ prefix, with the only exception that the result is returned instead of echoed. The existing functions are now simply wrappers echoing the output of the new get_*() functions.

This is in line with how other templating functions act.

[DrewAPicture]

Seems sensible to have a getter, though I'm a little torn on whether we need to introduce three new functions just so there's getter parity for both of them.

[mor10]

@DrewAPicture I think getter parity is the entire point here. I remember working with the existing functions here and having to do a lot of extra work just because the get_ functions were missing. There should be parity across different similar functions which is what @flixos90 is proposing.