Add new C++ Command framework (#1785)

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.

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

@@ -0,0 +1,34 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <frc/smartdashboard/Sendable.h>
+
+#include <string>
+
+#include "Subsystem.h"
+
+namespace frc2 {
+/**
+ * A base for subsystems that handles registration in the constructor, and
+ * provides a more intuitive method for setting the default command.
+ */
+class SubsystemBase : public Subsystem, public frc::Sendable {
+ public:
+  void InitSendable(frc::SendableBuilder& builder) override;
+  std::string GetName() const override;
+  void SetName(const wpi::Twine& name) override;
+  std::string GetSubsystem() const override;
+  void SetSubsystem(const wpi::Twine& name) override;
+  void AddChild(std::string name, frc::Sendable* child);
+
+ protected:
+  SubsystemBase();
+  std::string m_name;
+};
+}  // namespace frc2