- added drag/drop support

- added drag/drop into Interact - events for dropactivate, dropdeactive,
dropenter, dropleave, dropover and drop
- Interact now has dropzone(accept: string), which accepts a selector,
if we are moving a selector of that type from any other Interact, then
we start the dropzone events and logic
- added dx,dy to the move event
- added stack and fan layouts
- changed padding to offsetx and offsety for layouts
- added examples for random layout and drag and drop