[command] Add PIDSubsystem PIDController as child (#2784)

Previously, the PIDSubsystem's PID Controller would show as ungrouped in
LiveWindow.

Fixes wpilibsuite/RobotBuilder#260

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

@@ -12,6 +12,7 @@ using namespace frc2;
 PIDSubsystem::PIDSubsystem(PIDController controller, double initialPosition)
     : m_controller{controller} {
   SetSetpoint(initialPosition);
+  AddChild("PID Controller", &m_controller);
 }
 
 void PIDSubsystem::Periodic() {