rework pagination - fixes #491

previously, when adding items either at the beginnig (e.g. new notes
coming in while we're not looking at the top of the timeline) or a the
end (e.g. more items arriving from a background fetch) of a paginated
view, the resulting list got truncated to `displayLimit`, potentially
throwing data away and causing a new fetch.

This, coupled with the async nature of scrolling & fetching, could
cause weird results.

Also, `offset` was always incremented by the size of the fetched
results, even if not all of them were displayed, meant that it was
possible for offset-based pagination to drop items.

Finally, the "queue" of new items (usually, new notes) also got
truncated to `displayLimit`, which again could drop items (this effect
was usually masked by the first point: when scrolling to the top of
the timeline, if the queue's length was equal to `displayLimit`, those
notes displaced any existing ones, `unshiftItems` set
`more.value=true`, you got scrolled to the top, and notes were fetched
again, so you lost your position but at least all notes got shown,
eventually)