DOING: Dirk fix tons of stuff

this PR fixes several issues:

- clicking closing cross kills content for omnibox: https://github.com/nens/lizard-nxt/issues/430
- inconsistent rendering of zoom buttons between browsers
- generic solution for preventing "null-only" API reponses filling the omnibox: https://github.com/nens/lizard-nxt/issues/427
- non-generic hiding of timeseries with too little data
- remove header text for structure attribute table: https://github.com/nens/lizard-nxt/issues/428
- omnibox cards will no longer stay transparent after map is finished moving.
- repair CSV export for rain: https://github.com/nens/lizard-nxt/issues/402
- prevent timeState.at becoming 'null': https://github.com/nens/lizard-nxt/issues/419
- line-tool: prevent unneccesary 3rd click after first two clicks don't resolve