Improve input box startup UX

[nwittwer]

Problem
When you start up the app, it currently doesn't load anything. You just see some empty boxes.

Expectation
"What should I do?". The expectation is that something should load, and they can change it if they'd like. If they've used the app before, it just loads the site they last visited.

Checklist

• Check if recent URL(s) in localStorage exist
  • If so, insert the most recent one in the URL input box
    • Load that URL

[nwittwer]

• Added an illustration and helper text to canvas when there's no screens
• Added animated tooltip to make it clear where to click to create a new screen
• URL input box is only shown once a screen is created
• The previous URL and all screen sizes are automatically loaded on startup

These improvements should make it much more clear how to get started.