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;