Tilemap UI improvements

[thomasneirynck]

Much of the features of the tilemap application should be improved. This is a meta-ticket that collects some additions/breaking changes we could consider doing together. Most of these impact/alter the existing behavior.

Change UI.

• the heatmap controls are much too specialized. These should be replaced by a more simple UI (Simplify heatmap controls #11759)
• the precision selection (auto vs manual) is unnecessary. Consider replacing with something more intuitive (Add ability to modify selection of geohash precision on a sliding scale #10469)

Modify behavior:

• modify filtering behavior (Tilemap bounding box filter should modify the existing filter #3731): instead of adding filters to the filter bar on each selection, consider replacing the filters with the new selection.
• Requests data in bounds (Filter geohash_grid aggregation to map view box with collar #8087): only request data within the visible bounds

New features:
- Visualize two metrics: (#10447): allow ability to visualize multiple metrics (this does not work for heatmaps and might require some re-architecting of the UI)

[thomasneirynck]

For the initial 6.0 release, we should probably hold off on introducing new features, and focus on deprecation and changes in behavior.

[thomasneirynck]

Closing. Resolved #11759 and #8087. May address others, but they are more nice-to-haves.