Add annotation processors to register classes and compile resources

[jwharm]

Implement an annotation processor to find @RegisteredType- and @GtkTemplate-annotated classes, and save the names in a resource file. On startup, the static class initializers in the global GObject and Gtk classes will read the resource file, and automatically register the types as GObject classes.

A new TypeCache method will be necessary to retrieve the gtype for a Class<? extends GObject>. (Currently the TypeCache only supports the reverse.)

Taking the idea a step further, an annotation processor could trigger a GResource compile step for UI files that are used in Gtk composite template classes. This would allow users to define the UI as an annotation inside the Java file. It would even be possible to call the Blueprint compiler automatically if the annotation processor recognizes a blueprint template.