deleteCurrentLayer () {
    if (this.current_layer && this.getNumLayers() > 1) {
      const oldLayerGroup = this.current_layer.removeGroup()
      this.identifyLayers()
      return oldLayerGroup
    }
    return null
  }
Deletes the current layer from the drawing and then clears the selection.
This function then calls the 'changed' handler.  This is an undoable action.
@todo Does this actually call the 'changed' handler?
@returns {SVGGElement} The SVGGElement of the layer removed or null.