do not rebuild whole collection on download

just enter the articles at the right indices already during
download. This is faster n*k instead of n*log(n).
delay archiving, till user leaves article view.

Hm.. don't like it, yet... probably still doBack()
in the archiveTap, but delay it a few secs to let user
re-add?

Not sure how to handle that. Maybe make preference?
increased timeout for initial scroll a little.
try to store article read pos on app close

somehow onunload never get's called, though.. don't know why.
fix article download for not-yet-refreshed articles.
fixed articleModel not notified about failed download

ARHG.. this took MUCH too long to notice. ;)
store collection on unload

doesn't work here, either...
made goback on archive optional.