Remove uses of deprecated APIs

Depends on atom/atom@a6f31ed79192cba8f1509a819fd2689e3f050c93 being released.
> Do you think it'd be better to re-use the panel?

Both seem to work and I don't know a downside to this approach except more DOM element get created on show which possibly could slow down bigger views. But then leaving the panel there after close could leave lots of DOM elements around that could possibly bloat the DOM as well which I'm not sure what the impact of that will be.

Either way, it would probably be good to solidify on one or the other so the bundled packages are consistent examples of ways we recommend using panels.