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/drive/DifferentialDrive.cpp

@@ -211,6 +211,8 @@ void DifferentialDrive::GetDescription(wpi::raw_ostream& desc) const {
 
 void DifferentialDrive::InitSendable(SendableBuilder& builder) {
   builder.SetSmartDashboardType("DifferentialDrive");
+  builder.SetActuator(true);
+  builder.SetSafeState([=] { StopMotor(); });
   builder.AddDoubleProperty("Left Motor Speed",
                             [=]() { return m_leftMotor.Get(); },
                             [=](double value) { m_leftMotor.Set(value); });