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/Solenoid.cpp

@@ -87,6 +87,7 @@ void Solenoid::StartPulse() {
 
 void Solenoid::InitSendable(SendableBuilder& builder) {
   builder.SetSmartDashboardType("Solenoid");
+  builder.SetActuator(true);
   builder.SetSafeState([=]() { Set(false); });
   builder.AddBooleanProperty("Value", [=]() { return Get(); },
                              [=](bool value) { Set(value); });