Merge Workspace::stackingOrder() and Workspace::xStackingOrder()

The Workspace has two stacks - one with managed windows and deleted
windows, the other includes windows from the first stack + override
redirect windows.

This change merges both stacks. It has several benefits - we will be
able to move window elevation stuff to Workspace and streamline the
scene stuff, for example it will be possible to have a root item.
Another advantage is that unmanaged windows will have
Window::stackingOrder() property set, which can be useful in the future
in qml effects or (qtquick scene if we push harder in that front).
Another advantage is that kwin will make less X11 calls when restacking
managed windows.