Fix C++ SendableRegistry::AddChild() (#2207)

Add a Sendable* overload so pointers to sendable objects work appropriately.
Otherwise an AddLW(this) in a child (which is a Sendable*) could be a
different pointer than a void* to the same object.

For example:
  AnalogInput constructor calls AddLW(this)
  AnalogPotentiometer constructor calls AddChild(analog input pointer)

Also add handling for the child object moving (if it's Sendable).

wpilibc/src/main/native/include/frc/smartdashboard/SendableRegistry.h

@@ -118,6 +118,15 @@ class SendableRegistry {
   void AddLW(Sendable* sendable, const wpi::Twine& subsystem,
              const wpi::Twine& name);
 
+  /**
+   * Adds a child object to an object.  Adds the child object to the registry
+   * if it's not already present.
+   *
+   * @param parent parent object
+   * @param child child object
+   */
+  void AddChild(Sendable* parent, Sendable* child);
+
   /**
    * Adds a child object to an object.  Adds the child object to the registry
    * if it's not already present.