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/SPI.h

@@ -9,15 +9,12 @@
 
 #include <stdint.h>
 
-#include "SensorBase.h"
+#include "ErrorBase.h"
 
 enum HAL_SPIPort : int32_t;
 
 namespace frc {
 
-class DigitalOutput;
-class DigitalInput;
-
 /**
  * SPI bus interface class.
  *
@@ -25,12 +22,12 @@ class DigitalInput;
  * It probably should not be used directly.
  *
  */
-class SPI : public SensorBase {
+class SPI : public ErrorBase {
  public:
   enum Port { kOnboardCS0 = 0, kOnboardCS1, kOnboardCS2, kOnboardCS3, kMXP };
 
   explicit SPI(Port port);
-  virtual ~SPI();
+  ~SPI() override;
 
   SPI(const SPI&) = delete;
   SPI& operator=(const SPI&) = delete;