added the new function SwapEntityGroup to be able to easily move entities between groups

split EngineRoot for better readability and add comments

- some renaming

- Support for grouped nodes (adding and querying them) is almost complete
- EngineRoot and EngineNodeDB can now support nodes as struct and class
- INodeEngine Add and Remove functions accept only NodeWithID (classes only)
-

- clean up
- the structnode ids must be int

Alpha

- FasterList Clear has been renamed to FastClear. The standard Clear will clean the array like the normal List does
- An engine can now receive callback for when nodes are disabled and enabled
-

- improve NETFX_CORE compatibility
- implementors can now be added after the descriptor is created
- SingleNodeEngine is now abstract

This reverts commit 4089cd7309.

Alpha

# Conflicts:
#	ECS/EnginesRoot.cs