Implement sticky windows

The implementation works like this:
Containers can have a 'sticky-group' attribute. Imagine two different
containers (on two different workspaces) which have the same sticky-group.
Now you open a window in the first container. When you switch to the
other workspace, the window will be re-assigned to the other container.

An obvious problem which is not covered with the code at the moment is
having two containers with the same sticky-group visible at the same time.

include/data.h

@@ -273,6 +273,11 @@ struct Con {
 
     char *name;
 
+    /* a sticky-group is an identifier which bundles several containers to a
+     * group. The contents are shared between all of them, that is they are
+     * displayed on whichever of the containers is currently visible */
+    char *sticky_group;
+
     /* user-definable mark to jump to this container later */
     char *mark;
 

include/x.h

@@ -12,6 +12,19 @@
  */
 void x_con_init(Con *con);
 
+/**
+ * Moves a child window from Container src to Container dest.
+ *
+ */
+void x_move_win(Con *src, Con *dest);
+
+/**
+ * Reparents the child window of the given container (necessary for sticky
+ * containers). The reparenting happens in the next call of x_push_changes().
+ *
+ */
+void x_reparent_child(Con *con, Con *old);
+
 /**
  * Re-initializes the associated X window state for this container. You have
  * to call this when you assign a client to an empty container to ensure that