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/DifferentialDrive.cpp

@@ -11,6 +11,7 @@
 
 #include <HAL/HAL.h>
 
+#include "SmartDashboard/SendableBuilder.h"
 #include "SpeedController.h"
 
 using namespace frc;
@@ -23,7 +24,13 @@ using namespace frc;
  */
 DifferentialDrive::DifferentialDrive(SpeedController& leftMotor,
                                      SpeedController& rightMotor)
-    : m_leftMotor(leftMotor), m_rightMotor(rightMotor) {}
+    : m_leftMotor(leftMotor), m_rightMotor(rightMotor) {
+  AddChild(&m_leftMotor);
+  AddChild(&m_rightMotor);
+  static int instances = 0;
+  ++instances;
+  SetName("DifferentialDrive", instances);
+}
 
 /**
  * Arcade drive method for differential drive platform.
@@ -249,3 +256,13 @@ void DifferentialDrive::StopMotor() {
 void DifferentialDrive::GetDescription(llvm::raw_ostream& desc) const {
   desc << "DifferentialDrive";
 }
+
+void DifferentialDrive::InitSendable(SendableBuilder& builder) {
+  builder.SetSmartDashboardType("DifferentialDrive");
+  builder.AddDoubleProperty("Left Motor Speed",
+                            [=]() { return m_leftMotor.Get(); },
+                            [=](double value) { m_leftMotor.Set(value); });
+  builder.AddDoubleProperty("Right Motor Speed",
+                            [=]() { return m_rightMotor.Get(); },
+                            [=](double value) { m_rightMotor.Set(value); });
+}