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

RELEASE-NOTES-next

@@ -35,6 +35,7 @@ option is enabled and only then sets a screenshot as background.
   • i3bar: use first bar config by default
   • i3-dump-log -f now uses UNIX sockets instead of pthreads. The UNIX socket approach
     should be more reliable and also more portable.
+  • Implement the include config directive
   • Allow for_window to match against WM_CLIENT_MACHINE
   • Add %machine placeholder (WM_CLIENT_MACHINE) to title_format
   • Allow multiple output names in 'move container|workspace to output'