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/libc_compat/libc_compat.c

@@ -21,6 +21,10 @@
  * incompatible with the system libraries that Blender is built on. To solve
  * this we add a few -ffast-math symbols that can be missing. */
 
+/** \file
+ * \ingroup intern_libc_compat
+ */
+
 #ifdef __linux__
 #  include <features.h>
 #  include <math.h>