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/Relay.cpp

@@ -201,6 +201,7 @@ void Relay::GetDescription(wpi::raw_ostream& desc) const {
 
 void Relay::InitSendable(SendableBuilder& builder) {
   builder.SetSmartDashboardType("Relay");
+  builder.SetActuator(true);
   builder.SetSafeState([=]() { Set(kOff); });
   builder.AddSmallStringProperty(
       "Value",