Add handle moved signal to split panel

[afshin]

The handleMoved signal is useful for capturing state changes in the layout of a SplitPanel and saving it (e.g. for layout restoration). This PR allows for removing the following code in JupyterLab:

import { ISignal, Signal } from '@lumino/signaling';
import { SplitPanel as SPanel } from '@lumino/widgets';

/**
 * A deprecated split panel that will be removed when the phosphor split panel
 * supports a handle moved signal. See https://github.com/phosphorjs/phosphor/issues/297.
 */
export class SplitPanel extends SPanel {
  /**
   * Emits when the split handle has moved.
   */
  readonly handleMoved: ISignal<any, void> = new Signal<any, void>(this);

  handleEvent(event: Event): void {
    super.handleEvent(event);

    if (event.type === 'mouseup') {
      (this.handleMoved as Signal<any, void>).emit(undefined);
    }
  }
}

https://github.com/jupyterlab/jupyterlab/blob/dbdefed9db9332381fe4104bdf53ec314451951f/packages/settingeditor/src/splitpanel.ts

[fcollonval]

Thanks @afshin