Change how cascading works

[BTMorton]

At the moment cascading is not the best process. This should be improved. At the moment it follows the style of http://gridster.net/ but it would be good if it also supports the following cascade style:

Basically, it will fill in gaps in the grid rather than just shifting everything down.

Additionally, it would be more useful if the placeholder position represents where the item will actually be dropped, rather than just being under the item as it is now.

[ShipraGrover]

If we can have grid cascade this way it will be great