Chore/move gui to modules (#3001)

Move `GUI` to esm

Author: chmelevskij

.gitignore

@@ -31,3 +31,4 @@ nbproject/
 # Eclipse
 .project
 .settings/
+test-results-junit/

src/js/Clipboard.js

@@ -1,4 +1,4 @@
-'use strict';
+import GUI from './gui.js';
 
 /**
  * Encapsulates the Clipboard logic, depending on web or nw
@@ -99,3 +99,5 @@ if (GUI.isNWJS()){
 } else {
     Clipboard._configureClipboardAsOther();
 }
+
+export default Clipboard;