Allow certain object types to occupy only specified layers

For example, I want to have a separate layer for lights, called "Lights", and a separate layer for enemies, called "Enemies". I can do that already, but it would be a nice further workflow improvement if Tiled prevented me to accidentally place an enemy type onto the "Lights" layer.

The allowed set of layers could be specified either in the object types window or in the object templates.

Start by inspecting the object types and object template workflows, then trace where object placement onto layers is validated. Define how allowed layers are configured and enforced so an enemy cannot be placed on the Lights layer, and verify the behavior for both object types and templates.

Written by the indexing model from the issue text.