Memory management improvements for Linux

[alice]

Note: I did this as a way to understand how the Linux code works, and to play with the idea I'd used for a similar job in Mac code. If we feel like this isn't actually an improvement, I'm happy to let it be!

• Wrap each AtspiNode's managed AtspiAccessible in a std::shared_ptr
• Add AtspiInterface<T> as a superclass for all the Atspi[Type]Interface classes.
  • Uses std::shared_ptr to manage references to wrapped objects, unref-ing when the Interface object goes out of scope
  • This means we don't need to worry about whether a particular query_interface method returns a reference to the existing object or a new object
• Add some convenience wrappers around std::unique_ptr to call the appropriate free/unref function, used for accessing data on AtspiAccessible interfaces

[alice]

@joanmarie I've love to get your thoughts on this as an idea, before I put it up for full review.