DataStore & History : Implement order/index methods

[nathanidp]

Currently the dataStore and the history don't manage the order of the paths and the entities but it should in a case where the order has its importance for history necessities (example z-index in 2D).

In order:
DataStore

• Add method that can get the index of a entity in its path
• Add method that exchange two entities

History

• Check the order difference when applying snapshot
• Reorder the entities