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/configuration.h

@@ -15,6 +15,7 @@
 #include "queue.h"
 #include "i3.h"
 
+typedef struct IncludedFile IncludedFile;
 typedef struct Config Config;
 typedef struct Barconfig Barconfig;
 extern char *current_configpath;
@@ -22,6 +23,7 @@ extern char *current_config;
 extern Config config;
 extern SLIST_HEAD(modes_head, Mode) modes;
 extern TAILQ_HEAD(barconfig_head, Barconfig) barconfigs;
+extern TAILQ_HEAD(includedfiles_head, IncludedFile) included_files;
 
 /**
  * Used during the config file lexing/parsing to keep the state of the lexer
@@ -69,6 +71,16 @@ struct Variable {
     SLIST_ENTRY(Variable) variables;
 };
 
+/**
+ * List entry struct for an included file.
+ *
+ */
+struct IncludedFile {
+    char *path;
+
+    TAILQ_ENTRY(IncludedFile) files;
+};
+
 /**
  * The configuration file can contain multiple sets of bindings. Apart from the
  * default set (name == "default"), you can specify other sets and change the