Split the two command implementations into separate libraries (#2012)

This will allow us at the user code side to determine to include old commands, new commands or both.

wpilibNewCommands/src/main/native/cpp/frc2/command/SubsystemBase.cpp

@@ -0,0 +1,66 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "frc2/command/SubsystemBase.h"
+
+#include <frc/smartdashboard/SendableBuilder.h>
+#include <frc/smartdashboard/SendableRegistry.h>
+#include <frc2/command/Command.h>
+#include <frc2/command/CommandScheduler.h>
+
+using namespace frc2;
+
+SubsystemBase::SubsystemBase() {
+  frc::SendableRegistry::GetInstance().AddLW(this, GetTypeName(*this));
+  CommandScheduler::GetInstance().RegisterSubsystem({this});
+}
+
+void SubsystemBase::InitSendable(frc::SendableBuilder& builder) {
+  builder.SetSmartDashboardType("Subsystem");
+  builder.AddBooleanProperty(".hasDefault",
+                             [this] { return GetDefaultCommand() != nullptr; },
+                             nullptr);
+  builder.AddStringProperty(".default",
+                            [this]() -> std::string {
+                              auto command = GetDefaultCommand();
+                              if (command == nullptr) return "none";
+                              return command->GetName();
+                            },
+                            nullptr);
+  builder.AddBooleanProperty(".hasCommand",
+                             [this] { return GetCurrentCommand() != nullptr; },
+                             nullptr);
+  builder.AddStringProperty(".command",
+                            [this]() -> std::string {
+                              auto command = GetCurrentCommand();
+                              if (command == nullptr) return "none";
+                              return command->GetName();
+                            },
+                            nullptr);
+}
+
+std::string SubsystemBase::GetName() const {
+  return frc::SendableRegistry::GetInstance().GetName(this);
+}
+
+void SubsystemBase::SetName(const wpi::Twine& name) {
+  frc::SendableRegistry::GetInstance().SetName(this, name);
+}
+
+std::string SubsystemBase::GetSubsystem() const {
+  return frc::SendableRegistry::GetInstance().GetSubsystem(this);
+}
+
+void SubsystemBase::SetSubsystem(const wpi::Twine& name) {
+  frc::SendableRegistry::GetInstance().SetSubsystem(this, name);
+}
+
+void SubsystemBase::AddChild(std::string name, frc::Sendable* child) {
+  auto& registry = frc::SendableRegistry::GetInstance();
+  registry.AddLW(child, GetSubsystem(), name);
+  registry.AddChild(this, child);
+}