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/cpp/Drive/MecanumDrive.cpp

@@ -13,6 +13,7 @@
 #include <HAL/HAL.h>
 
 #include "Drive/Vector2d.h"
+#include "SmartDashboard/SendableBuilder.h"
 #include "SpeedController.h"
 
 using namespace frc;
@@ -31,7 +32,15 @@ MecanumDrive::MecanumDrive(SpeedController& frontLeftMotor,
     : m_frontLeftMotor(frontLeftMotor),
       m_rearLeftMotor(rearLeftMotor),
       m_frontRightMotor(frontRightMotor),
-      m_rearRightMotor(rearRightMotor) {}
+      m_rearRightMotor(rearRightMotor) {
+  AddChild(&m_frontLeftMotor);
+  AddChild(&m_rearLeftMotor);
+  AddChild(&m_frontRightMotor);
+  AddChild(&m_rearRightMotor);
+  static int instances = 0;
+  ++instances;
+  SetName("MecanumDrive", instances);
+}
 
 /**
  * Drive method for Mecanum platform.
@@ -118,3 +127,19 @@ void MecanumDrive::StopMotor() {
 void MecanumDrive::GetDescription(llvm::raw_ostream& desc) const {
   desc << "MecanumDrive";
 }
+
+void MecanumDrive::InitSendable(SendableBuilder& builder) {
+  builder.SetSmartDashboardType("MecanumDrive");
+  builder.AddDoubleProperty("Front Left Motor Speed",
+                            [=]() { return m_frontLeftMotor.Get(); },
+                            [=](double value) { m_frontLeftMotor.Set(value); });
+  builder.AddDoubleProperty(
+      "Front Right Motor Speed", [=]() { return m_frontRightMotor.Get(); },
+      [=](double value) { m_frontRightMotor.Set(value); });
+  builder.AddDoubleProperty("Rear Left Motor Speed",
+                            [=]() { return m_rearLeftMotor.Get(); },
+                            [=](double value) { m_rearLeftMotor.Set(value); });
+  builder.AddDoubleProperty("Rear Right Motor Speed",
+                            [=]() { return m_rearRightMotor.Get(); },
+                            [=](double value) { m_rearRightMotor.Set(value); });
+}