Rework memory management

[jwharm]

This PR refactors a number of fundamental flaws in java-gi:

• Ownership handling does not belong in the proxy object constructor. It is now handled with explicit takeOwnership and yieldOwnership calls as specified in the GIR files.
• Split the Cleaner from ObjectBase into its own class
• Fixed ownership of callback return values: this is exactly opposite of regular methods
• Implemented much improved handling of allocated memory segments. All generated code that allocates memory is now wrapped in a try-with-resources statement to automatically release the memory afterwards. The only exception is the allocation of upcalll stubs: These are allocated in the global scope, and therefore never released.
• The memory allocation lifetimes introduced an extra indentation level, on top of the existing ones. This has been refactored so indentation is now inserted automatically after every newline. The indentation level is increased and decreased where necessary during code generation.
• Small fix for Union types. These inherited from ObjectBase but they are more akin to structs.

[jwharm]

The fix for ownership handling of callback return values also resolved the issue that I encountered in #20 .