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

docs/ipc

@@ -793,12 +793,44 @@ No payload.
 
 *Reply:*
 
-The config reply is a map which currently only contains the "config" member,
-which is a string containing the config file as loaded by i3 most recently.
+The config reply is a map which contains the following fields:
+
+config (string)::
+	The top-level config file contents that i3 has loaded most recently.
+	This field is kept for backwards compatibility. See +included_configs+
+	instead.
+included_configs (array of maps)::
+	i3 adds one entry to this array for each config file it loads, in
+	order. The first entry’s +raw_contents+ are identical to the +config+
+	field.
+
+Each +included_configs+ entry contains the following fields
+
+path (string)::
+	Absolute path name to the config file that i3 loaded.
+raw_contents (string)::
+	The raw contents of the file as i3 read them.
+variable_replaced_contents (string)::
+	The contents of the file after i3 replaced all variables. This is useful
+	for debugging variable replacement.
 
 *Example:*
 -------------------
-{ "config": "font pango:monospace 8\nbindsym Mod4+q exit\n" }
+{
+  "config": "include font.cfg\n",
+  "included_configs": [
+    {
+      "path": "/home/michael/configfiles/i3/config",
+      "raw_contents": "include font.cfg\n",
+      "variable_replaced_contents": "include font.cfg\n"
+    },
+    {
+      "path": "/home/michael/configfiles/i3/font.cfg",
+      "raw_contents": "set $font pango:monospace 8\nfont $font",
+      "variable_replaced_contents": "set pango:monospace 8 pango:monospace 8\nfont pango:monospace 8\n"
+    }
+  ],
+}
 -------------------
 
 [[_tick_reply]]