workspace: Don't try to use per-workspace MRU lists as a hint for focusing

Commit 2fc880db switched from focusing the topmost window as the default
window to focusing the MRU window. This was done in alignment with the
introduction of per-workspace MRU lists to avoid problems where the window
stack was inadvertently changed when focusing windows during window switches.

Now that focusing windows don't have as big an impact on the stacking order,
we can revert back to focusing the top window, which is less confusing to the
user.

For now, leave per-workspace MRU lists, as they're a pretty good approximation
of a global MRU list, and it works well enough.

https://bugzilla.gnome.org/show_bug.cgi?id=620744