Add support for drag&drop elements from the Toolbar onto a surface #56

It should be possible to set draggable on Toolbar items.
If you want all items to be draggable we need an appropriate property on the Toolbar itself for doing this.

Then there should be a callback for when a Toolbar item has been dropped with the Toolbar item instance being dropped.

On the Toolbar item we would like to optionally associate data with the item that can be used for identification.

Add specs and docs.