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/guardedalloc/cpp/mallocn.cpp

@@ -15,7 +15,7 @@
  */
 
 /** \file
- * \ingroup MEM
+ * \ingroup intern_mem
  */
 
 #include "../MEM_guardedalloc.h"