This enables compilation with llvm-clang and thus closes ticket #101.
While it makes the code more ugly, I don’t see a beautiful solution
which would enable us to stay with the more elegant solution of
nested functions and still allow compilation with any other compiler
than gcc.
Details which are missing: A command to hide/show all floating clients,
moving/resizing clients with your mouse holding Mod1 (click anywhere
in the client, not just on its borders), resize/move by keyboard, select
next/previous client by keyboard
