First cut at turning lists into typed variables

src/engine/variable.js

@@ -9,19 +9,27 @@ class Variable {
     /**
      * @param {string} id Id of the variable.
      * @param {string} name Name of the variable.
-     * @param {(string|number)} value Value of the variable.
+     * @param {(string|number)} type Type of the variable, one of "" or "list"
      * @param {boolean} isCloud Whether the variable is stored in the cloud.
      * @constructor
      */
-    constructor (id, name, value, isCloud) {
+    constructor (id, name, type, isCloud) {
         this.id = id || uid();
         this.name = name;
-        this.value = value;
+        this.type = type;
         this.isCloud = isCloud;
+        switch (this.type) {
+        case "":
+            this.value = 0;
+            break;
+        case "list":
+            this.value = [];
+            break;
+        }
     }
 
     toXML () {
-        return `<variable type="" id="${this.id}">${this.name}</variable>`;
+        return `<variable type="${this.type}" id="${this.id}">${this.name}</variable>`;
     }
 }