feat(perf): Add # of specific watchers & avg watcher time to perf pane

[SomeKittens]

The list of watchers in the performance pane now shows:

• Total time a particular watcher took in the last digest cycle
• The text of the watcher
• How many watchers of that type there are
• Avg time (total / how many)

closes #49

[btford]

LGTM