Bug#969159: darktable: embeds photoswipe

[Tino Mettler]

Hi,

I talked to some darktable developers. There are objections to include
an npm based project in darktable. One of them suggested to replace the
gallery export function by an own static gallery that is generated by a
Lua script.

He also created a simple PoF for that, I did the same. I posted the
current state on pixls.us to get feedback. My aim is implement all
features from the old PhotoSwipe gallery, plus some more, and get it
into one of the next darktable releases. The result will consist of
some static HTML and CSS files and a bit JavaScript code (a few hundred
LoC, compared to 4500 LoC in the PhotoSwipe variant).