windowManager: Update stacking during workspace switches

windowManager: Update stacking during workspace switches

When animating workspace switches, windows on the old and new workspaces
are temporarily reparented. If windows are restacked, those windows will
thus be ignored by mutter until meta_switch_workspace_completed() resyncs
the stacking at the end of the animation.
As a result, activating a window on another workspace that is not on top
of the stack is very noticeably a two-step operation of switching workspace
and raising the window. There is a technical reason for that order[0], but
we can avoid the visible disruption by manually syncing the stack during
the switch operation.

[0] https://git.gnome.org/browse/mutter/tree/src/core/workspace.c#n590

https://bugzilla.gnome.org/show_bug.cgi?id=741680