Implement 'workspace back_and_forth' (Patch by Michael Walle)

docs/userguide

@@ -737,6 +737,25 @@ force_xinerama yes
 Also note that your output names are not descriptive (like +HDMI1+) when using
 Xinerama, instead they are counted up, starting at 0: +xinerama-0+, +xinerama-1+, …
 
+=== Automatic back-and-forth when switching to the current workspace
+
+This configuration directive enables automatic +workspace back_and_forth+ (see
+<<back_and_forth>>) when switching to the workspace that is currently focused.
+
+For instance: Assume you are on workspace "1: www" and switch to "2: IM" using
+mod+2 because somebody sent you a message. You don’t need to remember where you
+came from now, you can just press mod+2 again to switch back to "1: www".
+
+*Syntax*:
+--------------------------------------
+workspace_auto_back_and_forth <yes|no>
+--------------------------------------
+
+*Example*:
+---------------------------------
+workspace_auto_back_and_forth yes
+---------------------------------
+
 == List of commands
 
 Commands are what you bind to specific keypresses. You can also issue commands
@@ -892,6 +911,10 @@ workspace 1, 3, 4 and 9 and you want to cycle through them with a single key
 combination. Similarily, you can use +move workspace next+ and +move workspace
 prev+ to move a container to the next/previous workspace.
 
+[[back_and_forth]]
+To switch back to the previously focused workspace, use +workspace
+back_and_forth+.
+
 To move a container to another xrandr output such as +LVDS1+ or +VGA1+, you can
 use the +move output+ command followed by the name of the target output. You
 may also use +left+, +right+, +up+, +down+ instead of the xrandr output name to
@@ -906,6 +929,9 @@ bindsym mod+2 workspace 2
 bindsym mod+Shift+1 move workspace 1
 bindsym mod+Shift+2 move workspace 2
 ...
+
+# switch between the current and the previously focused one
+bindsym mod+b workspace back_and_forth
 -------------------------
 
 ==== Named workspaces