removed "thumbnail" property from scene xml

src/store.js

@@ -1657,7 +1657,6 @@ Project.prototype.toXML = function (serializer) {
 
 Scene.prototype.toXML = function (serializer) {
     var tmp = new Scene(),
-        thumbdata,
         xml;
 
     function code(key) {
@@ -1676,19 +1675,11 @@ Scene.prototype.toXML = function (serializer) {
         return str;
     }
 
-    // catch cross-origin tainting exception when using SVG costumes
-    try {
-        thumbdata = this.thumbnail.toDataURL('image/png');
-    } catch (error) {
-        thumbdata = null;
-    }
-
     tmp.captureGlobalSettings();
     this.applyGlobalSettings();
     xml = serializer.format(
         '<scene name="@">' +
             '<notes>$</notes>' +
-            '<thumbnail>$</thumbnail>' +
             '<hidden>$</hidden>' +
             '<headers>%</headers>' +
             '<code>%</code>' +
@@ -1698,7 +1689,6 @@ Scene.prototype.toXML = function (serializer) {
             '</scene>',
         this.name || localize('Untitled'),
         this.notes || '',
-        thumbdata,
         Object.keys(StageMorph.prototype.hiddenPrimitives).reduce(
                 (a, b) => a + ' ' + b,
                 ''