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/SpeedControllerGroup.cpp

@@ -7,6 +7,8 @@
 
 #include "SpeedControllerGroup.h"
 
+#include "SmartDashboard/SendableBuilder.h"
+
 using namespace frc;
 
 void SpeedControllerGroup::Set(double speed) {
@@ -45,3 +47,10 @@ void SpeedControllerGroup::PIDWrite(double output) {
     speedController.get().PIDWrite(output);
   }
 }
+
+void SpeedControllerGroup::InitSendable(SendableBuilder& builder) {
+  builder.SetSmartDashboardType("Speed Controller");
+  builder.SetSafeState([=]() { StopMotor(); });
+  builder.AddDoubleProperty("Value", [=]() { return Get(); },
+                            [=](double value) { Set(value); });
+}