Refactor ScalarWidget

[Ingvord]

ScalarWidget MUST receive DataSource (Observable) and subscribe to it:

class ScalarWidget extends WaltzWidget {
  /**
  * @param {Observable<DataSource>} dataSource
  */
  constructor(dataSource, /*...*/){
     super(/*...*/)
     this.dataSource = dataSource;

     this.dataSource.subscribe(data => this.view.update(data))
  }

  /**
   * @return {webix.ui} this widgets UI
   */
  get view(){
     return $$(/*...*/);
  }
}

In the future all low level widgets should follow this pattern

Requires #246