Returns the version of Qooxdoo
@returns {String}
function() {
      if (this.__qooxdooVersion) {
        return this.__qooxdooVersion;
      }
      if (!this.__qooxdooVersion) {
         let lib = this.findLibrary("qx");
         if (lib) {
          this.__qooxdooVersion = lib.getVersion();  
         }
      }  
      return this.__qooxdooVersion;
    }