display top field values

It would be nice to have a way of seeing what values are available for a field on the left sidebar.
## What?
The sidebar should be able to display top values for a field without adding a widget onto the search page. Having to use a widget to get at the top field values is clunky at best, and limited to 15 values.
Additionally, I can't find a way to increase the widget beyond 15 values, and that's not helpful at all.

## Why?
We frequently drill down into fields where there are way more than 15 possible values (domains, URLs, sources, filenames, IP addresses). Right now there is no way to find more than the top 15. Even then, with the widget that gets added for top values, the interface feels very jarring as one goes back and forth between the sidepane and widgets. Then when you're done, you have to delete the widget, and go look for another field...

## Your Environment

* Graylog Version: 3.3.4