Update LiveWindow to provide continuous telemetry. (#771)

LiveWindow.updateValues() is now called from IterativeRobotBase on every
loop iteration.  Telemetry for all WPILib classes is enabled by default;
it can be disabled for specific classes using LiveWindow.disableTelemetry(),
or all telemetry can be disabled using LiveWindow.disableAllTelemetry().

This necessitated changing the hook methodology into other classes to
be more property-based rather than each class providing multiple functions.
This had the benefit of reducing boilerplate and increasing consistency.

- Remove NamedSendable, add name to Sendable.

- Provide SendableBase abstract class.

- Deprecate LiveWindow addSensor/addActuator interfaces.

- Add LiveWindow support to drive classes.

- Add addChild() helper functions to Subsystem.

- Fix inheritance hierarchy.  Now only sensors inherit from SensorBase.
  Other devices inherit from some combination of SendableBase, ErrorBase, or
  nothing.

wpilibc/src/main/native/include/Drive/MecanumDrive.h

@@ -67,7 +67,7 @@ class MecanumDrive : public RobotDriveBase {
   MecanumDrive(SpeedController& frontLeftMotor, SpeedController& rearLeftMotor,
                SpeedController& frontRightMotor,
                SpeedController& rearRightMotor);
-  virtual ~MecanumDrive() = default;
+  ~MecanumDrive() override = default;
 
   MecanumDrive(const MecanumDrive&) = delete;
   MecanumDrive& operator=(const MecanumDrive&) = delete;
@@ -79,6 +79,8 @@ class MecanumDrive : public RobotDriveBase {
   void StopMotor() override;
   void GetDescription(llvm::raw_ostream& desc) const override;
 
+  void InitSendable(SendableBuilder& builder) override;
+
  private:
   SpeedController& m_frontLeftMotor;
   SpeedController& m_rearLeftMotor;