Add pane titles to Timeline events, closes #239

[bkonkle]

I went with the suggested styling - 12px right-justified text - within an h2 element for semantics, since the headings are h3 elements.

[andyrichardson]

This is so subtle and yet makes a huge difference!

[andyrichardson]

Visual regression failing is expected (diffs are here).

Until we get something like chromatic, we have to update snapshots locally.

# Run visual regression
yarn visual-regression --watch

# Snapshots will fail - check the diff folder in `src/panel/__image_snapshots__`
# Assuming diffs are expected, hit 'u' to update snapshots 📸