workspace: Fade in instead of zoom to return desktop

The zooming animation of the windows looks nice when animating
from the workspace display page, but looks weird from others pages
like apps page or search page since the windows comes from nowhere
with a initial position not known for the user.

Instead of that just fade the desktop with the windows in its
original position.

https://bugzilla.gnome.org/show_bug.cgi?id=732901