workspace: Hide preview overlays from picks

Dragging and dropping app icons is expected to work anywhere over a
workspace, however overlaid elements are added to a separate hierarchy
and can thus block valid drop targets. This wasn't much of an issue
while we had just the window title, but since the addition of the
focus border, drops on window previews stopped working entirely.

Fix this by hiding all non-reactive overlay elements from picks.

https://bugzilla.gnome.org/show_bug.cgi?id=737166

js/ui/workspace.js

@@ -471,6 +471,10 @@ const WindowOverlay = new Lang.Class({
         this.title = title;
         this.closeButton = button;
 
+        // Don't block drop targets
+        Shell.util_set_hidden_from_pick(this.title, true);
+        Shell.util_set_hidden_from_pick(this.border, true);
+
         parentActor.add_actor(this.title);
         parentActor.add_actor(this.border);
         parentActor.add_actor(this.closeButton);