Code under a different license should be kept in a separate file. This
implemention is a single header file with ~65 lines, so it better fits this
requirement.

Fixes #276

Instead of rendering the entire filename at once, Xft will let us do it
character by character. This will allow sxiv to query fontconfig for
a font that can provide any missing codepoints, if needed.

A known issue of this patch is that the "..." dots rendering will not
work properly for very long multibyte filenames. That is because we
cannot easily predict the final width of the rendered filename before
drawing it. I couldn't figure out a clean way to deal with this, so I
ended up just truncating the offending filenames.

- Functions warn() and die() replaced by GNU-like error(3) function
- Register cleanup() with atexit(3)
- Functions called by cleanup() are marked with CLEANUP and are not allowed to
  call exit(3)

The function "free" performs input parameter validation.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/free.html

It is therefore not needed to check a passed pointer before this function call.
A corresponding update suggestion was generated by the software "Coccinelle"
from the following semantic patch approach.
http://coccinelle.lip6.fr/

@Remove_unnecessary_pointer_checks1@
expression x;
@@
-if (x != \(0 \| NULL\))
    free(x);

@Remove_unnecessary_pointer_checks2@
expression x;
@@
-if (x != \(0 \| NULL\)) {
    free(x);
    x = \(0 \| NULL\);
-}

@Remove_unnecessary_pointer_checks3@
expression a, b;
@@
-if (a != \(0 \| NULL\) && b != \(0 \| NULL\))
+if (a)
    free(b);

@Remove_unnecessary_pointer_checks4@
expression a, b;
@@
-if (a != \(0 \| NULL\) && b != \(0 \| NULL\)) {
+if (a) {
    free(b);
    b = \(0 \| NULL\);
 }

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>

- Configurable key and mouse mappings in config.h
- Put event handling code from main.c into events.[ch]