Improve editing EditorDock node

[KoBeWi]

• Adds property hint for default slot property.
  • Allow binding enum display names #106968 would be nice here.
• Adds icon for EditorDock class
  • It's editor-color MarginContainer, suggestions for better icons welcome.

While EditorDock can't be created directly (until #111518 at least), there are multiple ways to add it in your scene and it's quite useful. Improving the experience editing it makes sense.

[LunaCapra]

Perhaps an icon like this could work