[linearView] Cleanup workspace scrolling and zooming

Centralize the update of actor visibilities (overlays, shadows,
off-screen workspaces). Adjust the way scrolling is handled so
that it works correctly with removed workspaces.

https://bugzilla.gnome.org/show_bug.cgi?id=610191

js/ui/workspace.js

@@ -1143,6 +1143,9 @@ Workspace.prototype = {
     },
 
     _fadeInWindowOverlay: function(clone, overlay) {
+        if (clone.inDrag)
+            return;
+
         // This is a little messy and complicated because when we
         // start the fade-in we may not have done the final positioning
         // of the workspaces. (Tweener doesn't necessarily finish