add a draggedItemBuilder widget to support persistent state children

[danReynolds]

Adds a new draggedItemBuilder feature that resolves the GlobalKey issues described in #72. To persist the state of the children passed to widgets like ReorderableColumn and ReorderableRow, I was having issues with giving the children a GlobalKey or a GlobalObjectKey since the library internally wraps the children in a KeyedSubtree with a GlobalKey based on each child's Key.

This meant that there was no way to make persistent state children, something that the referenced issue was looking to solve and that I needed for my own usage. The solution I found was to build a different widget when dragging the child using a dragged item builder, since I can then give the children a GlobalKey and make them have persistent state without conflicting with the dragged item, which can be built with a non-persistent Key.

In addition to solving the persistent children state problem I think it's also a handy feature to have in case there are any other customizations folks want to make to a draggable widget while it is actively being dragged.

I'm happy to clean up the feature and make changes as advised. Let me know what you think thanks!

[diegotori]

LGTM.

[diegotori]

@danReynolds I'm currently out of the country, but I will release a new version next week. Thanks for the PR.

[TJ-coding]

@danReynolds
@diegotori
Thanks for implementing this fix, but I think there is still a bug.

Bottom Line Up First: ReorderableTableRow initialized with GlobalKey does not render correctly when picking it up by a long press. But it still reorder correctly when placed down.

1. When you long press a ReorderableTableRow initialized with an ObjectKey, the row hovers up.
   
2. This no longer happens when you initialize a ReorderableTableRow with a GlobalKey, and instead, it renders a very small shadow.
   
   

When GlobalKey is used, though the rendering when you pick up a row is incorrect, when you place the row, the reordering of a row that contains a StatefulWidget happens correctly.

However, when you set ObjectKey, though the rendering works correctly, the states of the StatefulWidgets in the row are reset to initial values.

I am aware that it takes a lot of effort to manage an open source project but I would like to know f you will be looking into fixing the bug. Unfortunately, I do not have the deep enough understanding of Flutter or this package to tackle this bug.

Thanks