Implement 'swap' command.

This patch introduces a new command 'swap' that swaps two containers so
that they assume each other's position and geometry.

fixes #917

docs/userguide

@@ -1976,6 +1976,39 @@ bindsym $mod+c move absolute position center
 bindsym $mod+m move position mouse
 -------------------------------------------------------
 
+=== Swapping containers
+
+Two containers can be swapped (i.e., move to each other's position) by using
+the +swap+ command. They will assume the position and geometry of the container
+they are swapped with.
+
+The first container to participate in the swapping can be selected through the
+normal command criteria process with the focused window being the usual
+fallback if no criteria are specified. The second container can be selected
+using one of the following methods:
+
++id+:: The X11 window ID of a client window.
++con_id+:: The i3 container ID of a container.
++mark+:: A container with the specified mark, see <<vim_like_marks>>.
+
+Note that swapping does not work with all containers. Most notably, swapping
+floating containers or containers that have a parent-child relationship to one
+another does not work.
+
+*Syntax*:
+----------------------------------------
+swap container with id|con_id|mark <arg>
+----------------------------------------
+
+*Examples*:
+-----------------------------------------------------------------
+# Swaps the focused container with the container marked »swapee«.
+swap container with mark swapee
+
+# Swaps container marked »A« and »B«
+[con_mark="^A$"] swap container with mark B
+-----------------------------------------------------------------
+
 === Sticky floating windows
 
 If you want a window to stick to the glass, i.e., have it stay on screen even