Bring back Zeek histogram

[philrz]

In the recent overhaul to the app workflow, the count() by _path histogram we've traditionally had for Zeek pools got dropped, so it needs to be restored. This work comes before the Generic Histogram that would cover non-Zeek pools (#1541).

[philrz]

Verified in Brim commit b08ceaf.

Some highlights as shown in the attached video:

1. The bar chart is not shown for data without a time range, such as the prs.json from the Zed tutorial. Supporting this visualization for generic data is tracked in Generic Histogram #1541.
2. Zeek data such as what's shown initially in the video includes the necessary time range, so the chart appears automatically in that case. This maintains feature parity for what existing Brim users have come to expect, such as for imported pcaps.
3. The whole time span of the pool is shown by default. If a Time Range pin is added, its initial values are set to this same whole time span.
4. The span of the Time Range pin can be further manipulated by brushing the chart or editing the pin settings directly.
5. The chart can be turned off if desired via the View menu

Verify.mp4

Thanks @jameskerr!