Add a way to indicate a Sendable is an actuator (#1226)

SendableBuilder.setActuator() sets the .actuator key in the network table
so dashboards can change behavior on the client side if desired, and also
sets a local flag (retrievable via isActuator()).

Both make drive bases actuators and call setSafeState on them.

wpilibc/src/main/native/cpp/PWMSpeedController.cpp

@@ -33,6 +33,7 @@ PWMSpeedController::PWMSpeedController(int channel) : SafePWM(channel) {}
 
 void PWMSpeedController::InitSendable(SendableBuilder& builder) {
   builder.SetSmartDashboardType("Speed Controller");
+  builder.SetActuator(true);
   builder.SetSafeState([=]() { SetDisabled(); });
   builder.AddDoubleProperty("Value", [=]() { return GetSpeed(); },
                             [=](double value) { SetSpeed(value); });