Lore Slides: more flexible behaviour for class='next' in <a> and <span>, remove element when link is invalid

[Nafai]

Various snippets of the IRC conversation:
<spiv> Nafai: Hmm, part of the problem is you *don't* want to list the title of
the prev/next slides in the hyperlink?
<spiv> Maybe I should change it so that <a class="next"> indicates that the href
attr should be set, but <span class="next"> indicates the title should be
appended to the element's text.
<spiv> Then you could do what you want, and the old behaviour would be done via
nesting, e.g. <a class="next"><span class="next">Next: </span></a>

<spiv> The no-tooltip would be achievable via deleting the element entirely in
the case that there's no prev (or next).

[spiv]

Here's a patch that should do this (support <a class="next"><span
class="next">Next: </span></a>).

Looking at the code, it appears that it should already delete an element with
class="next" if there's no next slide (and similarly for class="previous").

I guess this stuff needs test cases :/

Let me know how this goes.

[Nafai]

Patch seems to work fine and accomplish what I need.

[spiv]

Ok, great.  I've applied the patch.