Removed content-selection from _list/bbox-filter.
Removed emission of id->{doc:..} from _view/bbox.
Adjusted _list/bbox-filter to new _view/bbox KV pairs.

_view/bbox produces a neatly sorted key list now, so I threw away the poorly
coded bbox-filter. The new one runs (theoretically) faster, because it avoids
redundant bbox checks. It is also far moe readable than before.
Application avoids a viewport update if no more items are visible.

The simple reasoning is that if the last viewport contains less items than
requested, I don't have to update anything. Strangely, Chrome was confused by
the use of 'keys' in different scopes; capitalization fixed it somehow.