Adding "on demand" loading of a poster preview

[yarikoptic]

now that we have access to PDFs (I will soon populate git-annex of this repo with them) I wondered if it would be of any benefit to generate preview pngs for all of them, and then may be make them load/shown somehow, e.g. have a PREVIEW icon next to PDF with JS which would load up and show that png when hovered over, and disappear when moved away.

I am not feeling strongly about benefit of such functionality -- just want to throw against the wall if someone might find it interesting/useful