This directory contains "widgets" -- standalone reusable components. 

`sql` subdirectory contains "SQL widgets", which are responsible for fetching
the necessary data from the trace processor in addition to displaying it.

The current split between `ui/src/widgets` and `ui/src/frontend/widgets` (this
directory) is semi-arbitrary:
- `ui/src/widgets` contains low-level primitives (button, menu, etc) which
  depend only on //base.
- `ui/src/frontend/widgets` doesn't have dependency restrictions and contains
  higher-level primitives (e.g. SQL table viewer)

Both of these directories will be merged into one once the layering is sorted
out.