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/DigitalInput.h

@@ -8,10 +8,13 @@
 #pragma once
 
 #include "frc/DigitalSource.h"
+#include "frc/smartdashboard/Sendable.h"
+#include "frc/smartdashboard/SendableHelper.h"
 
 namespace frc {
 
 class DigitalGlitchFilter;
+class SendableBuilder;
 
 /**
  * Class to read a digital input.
@@ -22,7 +25,9 @@ class DigitalGlitchFilter;
  * as required. This class is only for devices like switches etc. that aren't
  * implemented anywhere else.
  */
-class DigitalInput : public DigitalSource {
+class DigitalInput : public DigitalSource,
+                     public Sendable,
+                     public SendableHelper<DigitalInput> {
  public:
   /**
    * Create an instance of a Digital Input class.