Custom data for layers

[afk-mario]

Currently there is a way to add custom data to levels and entities but not for layers, some examples of why this would be useful:

• Define the name of the material the layer is going to have for a custom shader per layer
• A collision mask or layer
• Currently the layer has a property for parallax info, couldn't find a way to change it from the UI but if there was a way to edit the layer data parallax info could live there.

[deepnight]

Duplicate #870