Add function to store layer control status (overlay only) in cookie for using in next session.

main.js

@@ -232,6 +232,10 @@ window.links = {};
 window.fields = {};
 window.resonators = {};
 
+// contain current status(on/off) of overlay layerGroups.
+// But you should use isLayerGroupDisplayed(name) to check the status
+window.overlayStatus = {};
+
 // plugin framework. Plugins may load earlier than iitc, so don’t
 // overwrite data
 if(typeof window.plugin !== 'function') window.plugin = function() {};