Support for nested sortable-group items.

I've run into a few issues when using `ember-sortable` with nested groups. My original approach assumed a child `sortable-group` `onChange` handler would be responsible for sorting child items. Unfortunately, it seems these change events bubble up to the handler for parent items. Once I realized that to be the case, I was able to work around the issue, by filtering out items accordingly by inspecting the draggedItem argument in the parent `onChange` callback. Would it be possible to scope change handlers to only worry about changes to direct `sortable-item` children? I believe this would make working with nested draggable lists a bit easier.

The second challenge I've run into is that the transitionY transform applied to items during drag seems to break down with nested `sortable-group`s. Despite explicitly setting `spacing` to 0 (though it seems this isn't required), a great deal of whitespace is inserted between all the draggable items in the tree (parent and child items). This makes for an unusable D&D experience, as it is difficult to drag an item past all the extra whitespace and still know where the item is going to be inserted relative to where it was previously.

### Original Approach

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### Screenshots

In this example, each category (i.e. Cherry, Apple, Peach) should be draggable to allow sorting of categories. In addition, each item within a category should also be draggable to allow sorting of those items (i.e. Cherry, Cherry With Fruit).