window-list: Avoid flashing when using auto-grouping

Currently when using auto-grouping, the list is ungrouped each
time a window is closed, and then possibly re-grouped on the next
allocation - as a result, there is a brief "ungroup flash" if the
list is supposed to remain grouped.
Avoid this by computing the width the ungrouped list would have
rather than by actually ungrouping it.

https://bugzilla.gnome.org/show_bug.cgi?id=738286