Root abstract interfaces

Following many comments received on modularity, Root version 2.23 includes a new set of abstract interfaces and a reorganisation of the libraries structure. This has been possible without breaking too much backward compatibility.
The abstract interfaces have two functions:
- Possibility for a user to redefine or extend a given implementation.
- Minimize dependencies between classes and shared libraries.

Following many comments received on modularity, Root version 2.23 includes a new set of abstract interfaces and a reorganisation of the libraries structure. This has been possible without breaking too much backward compatibility.