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/PWM.cpp

@@ -184,6 +184,7 @@ int PWM::GetChannel() const { return m_channel; }
 
 void PWM::InitSendable(SendableBuilder& builder) {
   builder.SetSmartDashboardType("PWM");
+  builder.SetActuator(true);
   builder.SetSafeState([=]() { SetDisabled(); });
   builder.AddDoubleProperty("Value", [=]() { return GetRaw(); },
                             [=](double value) { SetRaw(value); });