x11: Initialize opacity when starting to track Unmanaged
This is a minor regression that was introduced with the refactoring of Toplevel::opacity().
Previously, neither X11Client nor Unmanaged had to explicitly initialize the opacity because it was queried from the net info object in Toplevel::opacity().
With the refactored version, X11-specific opacity code was removed from the Toplevel class. When starting to manage a window, the opacity must be explicitly initialized.