Putting X windows and pointers to MetaWindows into a union had a number of
problems:

 - It caused awkward initialization and conditionalization
 - There was no way to refer to Wayland windows (represented by
   MetaWindow *) in the past, which is necessary for the MetaStackTracker
   algorithms
 - We never even cleaned up old MetaStackWindow so there could be
   records in MetaStackWindow pointing to freed MetaWindow.

Replace MetaStackWindow with a 64-bit "stack ID" which is:

 - The XID for X Windows
 - a "window stamp" for Wayland windows - window stamps are assigned
   for all MetaWindow and are unique across the life of the process.

https://bugzilla.gnome.org/show_bug.cgi?id=736559