GET_CONFIG: add raw/variable-processed contents of all config files (#4528)

We do this by adding to included_files as i3 processes the configs.

This should allow for easy debugging, without having to change how i3 processes
config files.

related to #4192

include/configuration.h

@@ -77,6 +77,8 @@ struct Variable {
  */
 struct IncludedFile {
     char *path;
+    char *raw_contents;
+    char *variable_replaced_contents;
 
     TAILQ_ENTRY(IncludedFile) files;
 };