Step drag and drop enhancement

The latest mabl trainer contains a number of enhancement to drag and drop of mabl steps within the trainer UI. Specifically:

1. If you select multiple steps and then drag one of them, all selected steps will be dropped at the new location.
2. When dragging, we now highlight the space between steps where the drop will occur, rather then the step being hovered over
3. You can drag steps to the trash can to delete them