(*) Now saving / loading a scene also saves / loads the geometric primitives of the scene (only creation and final model matrix). This implies the following major changes:
- Transformable class has been moved from client to common code.
- Added a new base type of commands (EntityCommand) and a new command type (ModelMatrixReplacementCommand) inheriting from previous EntityCommand.
- Added a "centroid" anttribute to GeometricPrimitiveCommand.
- Added methods for updating an entity's model matrix in all classes related to entities.
- Added code for processing entity commands in client and server.

(**) Part of the code of ResourcesOwnershipManager (server) has been duplicated into ResourcesSynchronizationLibrary. The idea is to fully integrate both classes and remove ResourcesOwnershipManager.