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/include/frc/smartdashboard/SendableBuilder.h

@@ -32,6 +32,14 @@ class SendableBuilder {
    */
   virtual void SetSmartDashboardType(const wpi::Twine& type) = 0;
 
+  /**
+   * Set a flag indicating if this sendable should be treated as an actuator.
+   * By default this flag is false.
+   *
+   * @param value   true if actuator, false if not
+   */
+  virtual void SetActuator(bool value) = 0;
+
   /**
    * Set the function that should be called to set the Sendable into a safe
    * state.  This is called when entering and exiting Live Window mode.