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/frc/DigitalOutput.h

@@ -10,10 +10,13 @@
 #include <hal/Types.h>
 
 #include "frc/ErrorBase.h"
-#include "frc/smartdashboard/SendableBase.h"
+#include "frc/smartdashboard/Sendable.h"
+#include "frc/smartdashboard/SendableHelper.h"
 
 namespace frc {
 
+class SendableBuilder;
+
 /**
  * Class to write to digital outputs.
  *
@@ -21,7 +24,9 @@ namespace frc {
  * elsewhere will allocate channels automatically so for those devices it
  * shouldn't be done here.
  */
-class DigitalOutput : public ErrorBase, public SendableBase {
+class DigitalOutput : public ErrorBase,
+                      public Sendable,
+                      public SendableHelper<DigitalOutput> {
  public:
   /**
    * Create an instance of a digital output.