Expose `emptyClick` event on GraferController

[DSheffield]

Presently you cannot subscribe to the 'emptyClick' event directly from the GraferController, as with other events (hover, hoverOff, click). Rather you have to dig deep in the controller internals, e.g.:

controller.viewport.graph.picking.on(UX.picking.PickingManager.events.emptyClick, someCallback))

This is not very clear / obvious. Would be simpler and more congruent with other events to do so directly from the controller:

controller.on(UX.picking.PickingManager.events.emptyClick, this.onBackgroundClick.bind(this));