Provide mechanism for a unique per-imageset gallery ID

[fluffy-critter]

Expected Behavior

Provide some mechanism for allowing each image set to have its own unique gallery ID. For example, if you have an entry like:

Entry-ID: 12345

![{256,256}](foo.jpg | bar.jpg | baz.jpg)

![{256,256}](qwer.jpg | poiu.jpg | asdf.jpg)

the resulting HTML would be something like:

<p><a href="foo.jpg" data-lightbox="12345.1"><img src="foo-256x256.jpg"></a>
<a href="bar.jpg" data-lightbox="12345.1"><img src="bar-256x256.jpg"></a>
<a href="baz.jpg" data-lightbox="12345.1"><img src="baz-256x256.jpg"></a></p>

<p><a href="qwer.jpg" data-lightbox="12345.2"><img src="qwer-256x256.jpg"></a>
<a href="poiu.jpg" data-lightbox="12345.2"><img src="poiu-256x256.jpg"></a>
<a href="asdf.jpg" data-lightbox="12345.2"><img src="asdf-256x256.jpg"></a></p>

Current Behavior

Having separate image sets requires hand-authoring the gallery ID.

Possible Solution

In the most general sense, allow the gallery option to take a Union[Callable,str] and if it's a Callable, call the function (but only at the start of the element render). Then provide a usable function like autoincrement that just provides an auto-incrementing gallery ID (as this doesn't have to be stable across pageviews, it just needs to be unique per lightbox on any given page).

Steps to Reproduce (for bugs)

Context