Map rules plugins

It will be nice to have a way to enforce custom rules while editing maps.

For example limit tilesets to be used by each specific layer, check map passability and so on. And be able to customize such constrains without touching main code base.

Currently I enforce such rules in export plugin, but it is too late sometimes ( because a lot of work already done on top of rule violation step )

Review how the existing export plugin enforces map rules and identify where rules could instead run during editing. Define the plugin configuration and validation behavior needed for examples such as tileset restrictions and passability checks; done means custom rules can prevent or flag violations before export without modifying the main codebase.

Written by the indexing model from the issue text.