This commit also introduces slurp() which reads a file in its entirety. Using
this function instead of doing IO in the functions in load_layout.c again and
again makes the code cleaner (fixing at least two memory leaks) and avoids
re-reading the same file 3 times.
related to #2755
Including config.h is necessary to get e.g. the _GNU_SOURCE define and
any other definitions that autoconf declares. Hence, config.h needs to
be included as the first header in each file.
This is done either via:
1. Including "common.h" (i3bar)
2. Including "libi3.h"
3. Including "all.h" (i3)
4. Including <config.h> directly
Also remove now-unused I3__FILE__, add copyright/license statement
where missing and switch include/all.h to #pragma once.
Michael Stapelberg