hacking-howto: Update 'data structures' section

- Includes updated bigpicture.png from dump-asy.pl script
- The X11 root window is not a container
- Adds some extra information

bigpicture.png is created by converting the .asy to an .eps with `asy
bigpicture .eps` and then using the following gs command:
gs -dSAFER -dBATCH -dNOPAUSE -dEPSCrop -r600 -sDEVICE=pngalpha -sOutputFile=bigpicture.png bigpicture.eps

docs/hacking-howto

@@ -246,37 +246,39 @@ A file containing all X11 atoms which i3 uses. This file will be included
 various times (for defining, requesting and receiving the atoms), each time
 with a different definition of xmacro().
 
-
 == Data structures
 
+See +include/data.h+ for documented data structures. The most important ones are
+explained here.
 
-See include/data.h for documented data structures. The most important ones are
-explained right here.
+The following picture is generated by the +contrib/dump-asy.pl+ script.
 
-/////////////////////////////////////////////////////////////////////////////////
-// TODO: update image
+image:bigpicture.png["The Big Picture",width=1000,link="bigpicture.png"]
 
-image:bigpicture.png[The Big Picture]
+The hierarchy is:
 
-/////////////////////////////////////////////////////////////////////////////////
-
-So, the hierarchy is:
-
-. *X11 root window*, the root container
-. *Output container* (LVDS1 in this example)
-. *Content container* (there are also containers for dock windows)
-. *Workspaces* (Workspace 1 in this example, with horizontal orientation)
-. *Split container* (vertically split)
-. *X11 window containers*
+. *Root container*
+. *Output containers*: +eDP-1+ in this example and the internal +__i3++ output
+. *Content and 2 dockarea containers*
+. *Workspaces*: Numbered workspace ``1'' and a ``Named workspace''
+. *Split containers*: One horizontal in the first workspace and a tabbed one in
+  the named one.
+. *Leaf containers*: Windows like vim and an i3bar dock.
 
 The data type is +Con+, in all cases.
 
-=== X11 root window
+=== Root container
 
-The X11 root window is a single window per X11 display (a display is identified
-by +:0+ or +:1+ etc.). The root window is what you draw your background image
-on. It spans all the available outputs, e.g. +VGA1+ is a specific part of the
-root window and +LVDS1+ is a specific part of the root window.
+The root container (global variable +croot+) is the up-most ascendant of every i3
+container. It can be used to iterate over the whole tree structure. E.g., it is
+used to reply to the +GET_WORKSPACES+ request, iterating over it's children to
+find all workspaces. This is different from the X11 root window.
+
+The X11 root window (global variable +root+) is a single window per X11 display
+(a display is identified by +:0+ or +:1+ etc.). The root window is what you draw
+your background image on. It spans all the available outputs, e.g. +VGA1+ is a
+specific part of the root window and +LVDS1+ is a specific part of the root
+window.
 
 === Output container
 
@@ -298,8 +300,8 @@ currently on.
 === Content container
 
 Each output has multiple children. Two of them are dock containers which hold
-dock clients. The other one is the content container, which holds the actual
-content (workspaces) of this output.
+the top and bottom dock clients. The other one is the content container, which
+holds the actual content (workspaces) of this output.
 
 === Workspace
 
@@ -318,10 +320,9 @@ vertical) and a layout.
 Split containers (and X11 window containers, which are a subtype of split
 containers) can have different border styles.
 
-=== X11 window container
+=== Leaf containers
 
-An X11 window container holds exactly one X11 window. These are the leaf nodes
-of the layout tree, they cannot have any children.
+A leaf container holds exactly one X11 window. They can't have any children.
 
 == List/queue macros