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

parser-specs/config.spec

@@ -20,6 +20,7 @@ state INITIAL:
   'set '                                   -> IGNORE_LINE
   'set	'                                  -> IGNORE_LINE
   'set_from_resource'                      -> IGNORE_LINE
+  'include'                                -> INCLUDE
   bindtype = 'bindsym', 'bindcode', 'bind' -> BINDING
   'bar'                                    -> BARBRACE
   'font'                                   -> FONT
@@ -63,6 +64,11 @@ state IGNORE_LINE:
   line
       -> INITIAL
 
+# include <pattern>
+state INCLUDE:
+  pattern = string
+      -> call cfg_include($pattern)
+
 # floating_minimum_size <width> x <height>
 state FLOATING_MINIMUM_SIZE_WIDTH:
   width = number
@@ -394,6 +400,8 @@ state BINDCOMMAND:
       ->
   command = string
       -> call cfg_binding($bindtype, $modifiers, $key, $release, $border, $whole_window, $exclude_titlebar, $command)
+  end
+      -> call cfg_binding($bindtype, $modifiers, $key, $release, $border, $whole_window, $exclude_titlebar, $command)
 
 ################################################################################
 # Mode configuration