Let the PlayerDropItemEvent work properly - Fixes BUKKIT-3313

As described in [BUKKIT-3313](https://bukkit.atlassian.net/browse/BUKKIT-3313), canceling a PlayerDropItemEvent moves the inHandItem to the first open slot. This PR fixes that, but not properly if an itemstack is moved outside of an open inventory window. In that case please use a InventoryClickEvent for canceling that.