Rename Workspace::restack() function
There are several ways how a window can be restacked relative to another: either by placing it below the reference window or above the reference window.
Currently, the restack() function places the given window below the refererence window. But it's hard to decipher from the name, one needs to take a look at the argument names to understand what's going on.
Another reason to rename is that it could be useful to have a stackAbove() function to make X11Window::restackWindow() implementation simpler and more straightforward.