trivial: Fix the signedness of boolean fields

The standard old kludge with gboolean being signed, not unsigned.
Encountered while printing the values for debugging.

https://bugzilla.gnome.org/show_bug.cgi?id=644306

src/shell-app.c

@@ -37,7 +37,7 @@ typedef struct {
   GSList *windows;
 
   /* Whether or not we need to resort the windows; this is done on demand */
-  gboolean window_sort_stale : 1;
+  guint window_sort_stale : 1;
 
   /* DBus property notification subscription */
   guint properties_changed_id : 1;