Replace MetaStackWindow with a 64-bit "stack ID"

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
MetaStackTracker: eliminate the resynchronization process

The step where we requery the stacking order from the server than combine
it in an arbitrary fashion with Wayland windows can be eliminated by observing
that we are the final authority for Wayland window stacking - so if we
apply each X event that we receive from the X server to our stack in a
way that leaves the X windows ordered in the same way as on the server,
and apply events that we have stored locally in a way that doesn't affect
the ordering of X windows, than we have a fully correct ordering of windows.

Ordering this in the order of first applying the X event and then applying the
local portion also means that as long as we had an up-to-date view of the X
stack the composite operation will be identical to what was requested.

https://bugzilla.gnome.org/show_bug.cgi?id=736559
Move manipulation of the X stack to MetaStackTracker

Since MetaStackTracker is the code that knows about the current X stacking order
and the relationship between X windows and Wayland windows, it's cleaner to
encapsulate stack manipulation in MetaStackTracker rather than have the calling
code make the X calls and only call into MetaStackTracker to inform it about
the changes.

https://bugzilla.gnome.org/show_bug.cgi?id=736559
Remove cache of last stacking order in stack.c

stack.c kept it's own record of the last stacking it requested, so that
restacking could be done with minimal moves, but we already have a better
view of the stacking order with the stack tracker, so use that instead.

This allows eliminating the special case for the first restack.

https://bugzilla.gnome.org/show_bug.cgi?id=736559
stack.c: remove obsolete handling of override-redirect windows

There was still code in stack.c to handle skipping override-redirect windows,
but since quite a while ago, meta_stack_add() is not called for OR windows
since they are outside our stacking control. Add an assertion and remove
unnecessary code.

https://bugzilla.gnome.org/show_bug.cgi?id=736559
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
MetaStackTracker: make functions used only internally static

Now that all actual stack shuffle is handled inside stack-tracker.c, we can make
meta_stack_tracker_record_[raise_above/lower_below] internal to that file and
remove the unused meta_stack_tracker_record_lower().

https://bugzilla.gnome.org/show_bug.cgi?id=736559
MetaStackTracker: optimize out unnecessary X restacking

We have a quite accurate view of the X stack, so there's no good reason to ask
the X server to do restacking that has no effect. (Restackings that have no
effect on either X windows or Wayland windows were generally optimized out in
the synchronization code, but in other cases like moving an X window that is
only beneath Wayland windows to the top of the stack we would make such
requests.)

Removing such requests:
 - Is a small efficiency win in itself
 - Allows us to immediately go ahead and apply Wayland changes to the verified stack
 - Prevents queued Wayland changes piling up waiting for an X event that will never
   be received, since the X server will not send confirmation of no-op restacks.

Since such operations may still have an effect on the relative stacking of X
and Wayland windows, we need to continue applying them to the local stack.

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