Sorting Support

[chadpod]

Figured I would issue this pull request since the feature is still being requested in the issue tracker on the repository. It adds drag/drop sorting, fixes a bug related to positioning the dragged item based on original grid index, and addresses some edge aliasing that was occurring for me during reorder. I've moved to mainly using collection views only, so I will probably delete my fork after this pull request is either merged or it becomes apparent that it won't be merged. Thanks for the great grid view!