Simplify Sendable interface (#1864)

This removes the name and subsystem from individual objects, and instead
puts this data into a new singleton class, SendableRegistry.  Much of
LiveWindow has been refactored into SendableRegistry.

In C++, a new CRTP helper class, SendableHelper, has been added to provide
move and destruction functionality.

Shims for GetName, SetName, GetSubsystem, and SetSubsystem have been added
to Command and Subsystem (both old and new), and also to SendableHelper to
prevent code breakage.

This deprecates SendableBase in preparation for future removal.

wpilibc/src/main/native/include/frc2/command/CommandScheduler.h

@@ -10,7 +10,8 @@
 #include <frc/ErrorBase.h>
 #include <frc/RobotState.h>
 #include <frc/WPIErrors.h>
-#include <frc/smartdashboard/SendableBase.h>
+#include <frc/smartdashboard/Sendable.h>
+#include <frc/smartdashboard/SendableHelper.h>
 
 #include <memory>
 #include <unordered_map>
@@ -34,7 +35,9 @@ class Subsystem;
  * with the scheduler using RegisterSubsystem() in order for their Periodic()
  * methods to be called and for their default commands to be scheduled.
  */
-class CommandScheduler final : public frc::SendableBase, frc::ErrorBase {
+class CommandScheduler final : public frc::Sendable,
+                               public frc::ErrorBase,
+                               public frc::SendableHelper<CommandScheduler> {
  public:
   /**
    * Returns the Scheduler instance.