A few visual tweaks

- overflow and white-space on ImportDialog .table elements, which
  prevents the .table-cell elements from being all awkward and such.
- Re-implemented projection ticks to aggregate by axis pixel. This
  reduces noise for large datasets and delivers a nice perf boost (since
  fewer elements need to be drawn.)

Next up: do some pixel-filtering on view-plot output (or maybe just
sample the data for now!)