Programmatic retrieval of layers' visibility always returns `true`

Hello again :)

Our team might have found a bug while using the Layer Manager API: every call to component.viewLayer.isVisible() seems to always return true, regardless of their actual state.

Steps to reproduce:

• Open the demo
• Switch to the layers panel
• Click on the eye icon to hide the "Body" layer
• Open the console
• Executing editor.getWrapper().viewLayer.isVisible() returns 1 instead of false

Looking at the code, I believe this.model.getStyle() should be used instead of this.model.get('style') (which always returns {}) to retrieve the current display value.

As always, I'd be happy to open a PR to fix it !

Cheers :)