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/SpeedControllerGroup.cpp

@@ -46,6 +46,7 @@ void SpeedControllerGroup::PIDWrite(double output) { Set(output); }
 
 void SpeedControllerGroup::InitSendable(SendableBuilder& builder) {
   builder.SetSmartDashboardType("Speed Controller");
+  builder.SetActuator(true);
   builder.SetSafeState([=]() { StopMotor(); });
   builder.AddDoubleProperty("Value", [=]() { return Get(); },
                             [=](double value) { Set(value); });