Implement include config directive (#4420)

The implementation uses wordexp(3) just like sway:
https://github.com/i3/i3/issues/1197#issuecomment-226844106

Thanks to jajm for their implementation at
bb55709d0a

This required refactoring the config parser to be re-entrant
(no more global state) and to return an error instead of dying.

In case a file cannot be opened, i3 reports an error but proceeds with the
remaining configuration.

Key bindings can be overwritten or removed using the new --remove flag of the
bindsym/bindcode directive.

All files that were successfully included are displayed in i3 --moreversion.

One caveat is i3 config file variable expansion, see the note in the userguide.

fixes #4192

include/config_directives.h

@@ -39,6 +39,7 @@ CFGFUN(criteria_init, int _state);
 CFGFUN(criteria_add, const char *ctype, const char *cvalue);
 CFGFUN(criteria_pop_state);
 
+CFGFUN(include, const char *pattern);
 CFGFUN(font, const char *font);
 CFGFUN(exec, const char *exectype, const char *no_startup_id, const char *command);
 CFGFUN(for_window, const char *command);