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

- 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.

DispatcherOnChange and DispatcherOnSet now hold weakreferences of the actions
Ticker is now deprecated, a legacy copy will be found in the example
Added Engines Profiler Tool
Added the concept of Entity Group (more will follow on the subject in an article)
Handling the creation of entities inside the Add Methods
fixed some bugs
renamed namespace to Svelto.ECS
add the concept of ExtraImplementors for the EntityDescriptor

the removal of the nodes will happen at the end of the frame and not immediately

Updated Context and Ticker Framework
Added Datastructures

Ticker and Observer become part of the ECS framework
Add support for Unity 4