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

testcases/t/201-config-parser.t

@@ -506,6 +506,7 @@ EOT
 
 my $expected_all_tokens = "ERROR: CONFIG: Expected one of these tokens: <end>, '#', '" . join("', '", 'set ', 'set	', qw(
         set_from_resource
+        include
         bindsym
         bindcode
         bind