Backgrounds and bug fixes

Changelog V1.0.2

• fixed an error that the plugin wouldn't work, when a not existing tech tree was referenced in notetags.
• added a warning when a tree is referenced in the notetags that isn't in the Techtree.json.
• added the ability to set custom background for trees and single nodes. This is done via the Techtree.json.
• added a folder for techtree backgrounds at ../img/system/techtree/bgimg/. All background images need to go there.
• updated the json schema for the techtree editor for custom backgrounds.
• added a plugin command to add a techtree to an actor.