Improvement: Add proper window properties

- Set WM_NAME, _NET_WM_NAME, WM_CLASS, and _NET_WM_WINDOW_TYPE on
  skippy-xd windows.

- Add Makefile recipe for src/.clang_complete.

- Wrap strdup() to mstrdup(), XFree() to sxfree(), XInternAtom() to
  get_atom(). Use macro to shorten wm_get_atoms(). Change _NET_WM_NAME
  to really point to _NET_WM_NAME instead of _NET_WM_VISIBLE_NAME.
  Correct return values. Indentation clean up, etc.