Make ol.interaction.DragAndDrop fire an 'addfeatures' event

This PR makes `ol.interaction.DragAndDrop` fire an `addfeatures` event instead of adding features to a specified source, or adding a layer to the map. This makes it possible to use it in different ways.

To demonstrate this, a drag-and-drop example that renders vector data using `ol.source.ImageVector`, which renders vector data to a single image layer entirely on the client.