Seems like some sort of alcohol based product would take it off. As an artist I make my own medium for oil painting and turpentine and tree sap are part of the main ingredients. The turpentine dissolves the sap so I would agree and say try the turpentine, just be sure to wear gloves and don't breath the fumes.
Although I don't know what effect it will have on the window. Maybe do a test on something else first.