Separate thumbnails out into var/thumbs. This clears up some ambiguity in Item_Model and simplifies

file_proxy.  It also means we can stop munging file names in the var/resizes hierarchy.

In the process, rename "thumbnail" to "thumb" everywhere in honor of
Chad (well, ok because it's shorter)..

core/libraries/Theme_View.php

@@ -105,9 +105,9 @@ class Theme_View_Core extends View {
     case "sidebar_top":
     case "tag_bottom":
     case "tag_top":
-    case "thumbnail_bottom":
-    case "thumbnail_info":
-    case "thumbnail_top":
+    case "thumb_bottom":
+    case "thumb_info":
+    case "thumb_top":
     case "photo_bottom":
       // @todo: restrict access to this option
       $debug = Session::instance()->get("debug", false);