Move NPCs, TileEvents and IOs specifications from map properties to marker objects

[jjppof]

Instead of setting NPCs, TileEvents and IOs in map properties, now the engine should support object markers from Tiled to represent them:

All the properties that were set in the string field in json format can now be set as separate property fields:

x and y positions should be inferred from object position. events_info, for instance, still would continue to be a string type with json content.

The object layers that would contain IOs, NPCs and TileEvents should have the following boolean properties:

• interactable_objects_layer
• npcs_layer
• tile_events_layer

[jjppof]

This one will have to wait for Tiled to support JSON data types.