Move logic for syncing the stack to the X server into MetaStackTracker

stack.c:sync_stack_to_xserver had both code for assembling the desired stack, and
code for enforcing the desired stack on the actual stack of X and Wayland windows;
the latter part is properly the domain of stack-tracker.c; moving the code to
apply the stack there both simplifies it and keeps stack.c more manageable.

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