Code generation for tree structure implementation

[raner]

Currently, Projo has no capabilities for creating correct implementations for tree-structured interface hierarchies, such as the ones created by the @Dtd annotation. The compile-time code creation will only create interfaces, and implementing fields as regular setters and getters will not produce the correct result.
Code generation should enable the actual creation/instantiation of interface hierarchies created by @Dtd and similar annotations. The actual conversion of the created tree structure (e.g., to XML or HTML) should be implementable in a base class that the generated interfaces all extend, and the API (if any additional API is needed) should be generic enough to allow creating of any structured target format.