Cleanup: reorganize doxygen modules

- Nest compositor pages under the compositor module
- Nest GUI, DNA/RNA & externformats modules under Blender.
- Remove modules from intern which no longer exist.
- Add intern modules (atomic, eigen, glew-mx, libc_compat, locale,
  numaapi, rigidbody, sky, utfconv).
- Use 'intern_' prefix for intern modules since some of the modules
  use generic terms such as locale & atomic.

intern/memutil/MEM_RefCountedC-Api.h

@@ -18,7 +18,7 @@
  */
 
 /** \file
- * \ingroup memutil
+ * \ingroup intern_memutil
  *
  * Interface for C access to functionality relating to shared objects in the foundation library.
  */