Commit bb884cb6 introduced the faulty behavior where
NautilusCanvasView keeps scrolling back to the first
selected item when selecting multiple items.

Backtracing it, it was found out that the icon view
was changing the "sort" state when selecting items,
even if nothing actually changed, which triggers the
sort action changed handler. The handler by itself
was doing the right job by revealing the selection,
and the issue was caused by calling it unnecessarily.

Since we may have to update the action states for other
reasons besidse real sort change, we must change the
action state only when it makes sense, i.e. only when
the states really change.

https://bugzilla.gnome.org/show_bug.cgi?id=748265