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.

wpilibcExamples/src/main/cpp/examples/SelectCommand/include/RobotContainer.h

@@ -0,0 +1,50 @@
+/*----------------------------------------------------------------------------*/
+/* 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 <frc2/command/Command.h>
+#include <frc2/command/PrintCommand.h>
+#include <frc2/command/SelectCommand.h>
+
+/**
+ * This class is where the bulk of the robot should be declared.  Since
+ * Command-based is a "declarative" paradigm, very little robot logic should
+ * actually be handled in the {@link Robot} periodic methods (other than the
+ * scheduler calls).  Instead, the structure of the robot (including subsystems,
+ * commands, and button mappings) should be declared here.
+ */
+class RobotContainer {
+ public:
+  RobotContainer();
+
+  frc2::Command* GetAutonomousCommand();
+
+ private:
+  // The enum used as keys for selecting the command to run.
+  enum CommandSelector { ONE, TWO, THREE };
+
+  // An example selector method for the selectcommand.  Returns the selector
+  // that will select which command to run.  Can base this choice on logical
+  // conditions evaluated at runtime.
+  CommandSelector Select() { return ONE; }
+
+  // The robot's subsystems and commands are defined here...
+
+  // An example selectcommand.  Will select from the three commands based on the
+  // value returned by the selector method at runtime.  Note that selectcommand
+  // takes a generic type, so the selector does not have to be an enum; it could
+  // be any desired type (string, integer, boolean, double...)
+  frc2::SelectCommand<CommandSelector> m_exampleSelectCommand{
+      [this] { return Select(); },
+      // Maps selector values to commands
+      std::pair{ONE, frc2::PrintCommand{"Command one was selected!"}},
+      std::pair{TWO, frc2::PrintCommand{"Command two was selected!"}},
+      std::pair{THREE, frc2::PrintCommand{"Command three was selected!"}}};
+
+  void ConfigureButtonBindings();
+};