Design insert/move operations on top of ordered (leaf-)lists

In order to implement MDSAL-246 we need a way how to express insert/move operations within ordered lists.

This should be a two-step process, with this issue dealing with the frontend operations and internal representation, i.e. how does the user express the operations and how they are tracked and applied within InMemoryDataTree.

The results of these operations can result in straight overwrites being reported by the DataTreeCandidate and improving the situation there should be a separate effort.