Live previews for hidden windows.

ClutterActors for hidden windows (such windows on different than active
workspaces and windows that are minimized) are available, and reflect the
actual state of the window. This is intended for use in task-switchers etc.

This feature is disabled by default (due to increased demand on resources),
and can be enabled through the metacity/general/live_hidden_windows gconf key.

A trivial sample window switcher is included in the scratch plugin (activated
by clicking on the slide out panel).

src/core/stack.c

@@ -240,7 +240,13 @@ get_standalone_layer (MetaWindow *window)
 {
   MetaStackLayer layer;
   gboolean focused_transient = FALSE;
-  
+
+  if (window->hidden)
+    {
+      layer = META_LAYER_DESKTOP;
+      return layer;
+    }
+
   switch (window->type)
     {
     case META_WINDOW_DESKTOP:
@@ -329,7 +335,8 @@ compute_layer (MetaWindow *window)
    * windows getting in fullscreen layer if any terminal is
    * fullscreen.
    */
-  if (WINDOW_HAS_TRANSIENT_TYPE(window) &&
+  if (window->layer != META_LAYER_DESKTOP &&
+      WINDOW_HAS_TRANSIENT_TYPE(window) &&
       (window->xtransient_for == None ||
        window->transient_parent_is_root_window))
     {
@@ -854,7 +861,6 @@ stack_do_relayer (MetaStack *stack)
           meta_topic (META_DEBUG_STACK,
                       "Window %s moved from layer %u to %u\n",
                       w->desc, old_layer, w->layer);
-              
           stack->need_resort = TRUE;
           stack->need_constrain = TRUE;
           /* don't need to constrain as constraining