Moves all CommandBase functionality into Command and deprecates CommandBase for removal.
Moves all SubsystemBase functionality into Subsystem and deprecates SubsystemBase for removal.
Adds a function to CommandScheduler to remove all registered Subsystems.
This is the C++ version of #1682.
The old command framework is still available, but will be deprecated.
Due to name conflicts, the new framework is in the frc2 namespace.
Eventually (after the old command framework is removed in a future year)
it will be moved into the main frc namespace.
The old headers were moved into folders because doing so avoids polluting
the system include directories.
Folder names were also normalized to lowercase.
