Replace WP button with a Frontenberg button

[tomjn]

I'd like to use this:

https://github.com/WordPress/gutenberg/blob/086b77ed409a70a6c6a6e74dee704851eff812f2/docs/reference-guides/slotfills/main-dashboard-button.md

To swap out the WP button in the top left corner and replace it with a panel that has links here and elsewhere