Put documentation for each function in the header files, doxygen-compatible

Put documentation for each function in the header files, doxygen-compatible

Thanks to psychoschlumpf for the hint. Having comments in the headers
makes it easier to get the big picture when not being interested in the
source.

The doxygen file extracts as much as it can into HTML files. Please note
that this is not official/supported documentation, but rather being nice
to people who have to/want to use doxygen (I don’t).

Let me paste the header of the doxygen file:

  You can use this file with doxygen to create a pseudo-documentation
  automatically from source. doxygen-comments are not used very extensively
  in i3, mostly for the reason that it clutters the source code and has no
  real use (doxygen’s output looks really ugly).

  So, if you want to use it, here you go. This is however not a supported
  document, and I recommend you have a look at the docs/ folder or at
  http://i3.zekjur.net/ for more, real documentation.