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/DifferentialDrive.h

@@ -102,7 +102,7 @@ class DifferentialDrive : public RobotDriveBase {
   static constexpr double kDefaultQuickStopAlpha = 0.1;
 
   DifferentialDrive(SpeedController& leftMotor, SpeedController& rightMotor);
-  virtual ~DifferentialDrive() = default;
+  ~DifferentialDrive() override = default;
 
   DifferentialDrive(const DifferentialDrive&) = delete;
   DifferentialDrive& operator=(const DifferentialDrive&) = delete;
@@ -118,6 +118,8 @@ class DifferentialDrive : public RobotDriveBase {
   void StopMotor() override;
   void GetDescription(llvm::raw_ostream& desc) const override;
 
+  void InitSendable(SendableBuilder& builder) override;
+
  private:
   SpeedController& m_leftMotor;
   SpeedController& m_rightMotor;