Rearrange List-Items in the Editor #759

Summary

It think at some points, it would be useful to rearrange (move up, move down) items in a list.

Example scenario:
Creating a custom render setup: If you want to add a render step between existing render steps, you have to create a new list and do all the steps again. meh.

Here is a screenshot, how we could implement this. Adding two buttons (Up and Down) in the object inspector. As soon as the user selects an item in the list, the two buttons gets enabled.