window-tracker: Use MetaWindow:skip-taskbar

The code from shell_window_tracker_is_window_interesting() is equivalent
of MetaWindow's skip-taskbar property, so use it to avoid code duplication.

https://bugzilla.gnome.org/show_bug.cgi?id=723308

src/shell-window-tracker.c

@@ -144,37 +144,7 @@ shell_window_tracker_class_init (ShellWindowTrackerClass *klass)
 gboolean
 shell_window_tracker_is_window_interesting (MetaWindow *window)
 {
-  if (meta_window_is_override_redirect (window)
-      || meta_window_is_skip_taskbar (window))
-    return FALSE;
-
-  switch (meta_window_get_window_type (window))
-    {
-      /* Definitely ignore these. */
-      case META_WINDOW_DESKTOP:
-      case META_WINDOW_DOCK:
-      case META_WINDOW_SPLASHSCREEN:
-      /* Should have already been handled by override_redirect above,
-       * but explicitly list here so we get the "unhandled enum"
-       * warning if in the future anything is added.*/
-      case META_WINDOW_DROPDOWN_MENU:
-      case META_WINDOW_POPUP_MENU:
-      case META_WINDOW_TOOLTIP:
-      case META_WINDOW_NOTIFICATION:
-      case META_WINDOW_COMBO:
-      case META_WINDOW_DND:
-      case META_WINDOW_OVERRIDE_OTHER:
-        return FALSE;
-      case META_WINDOW_NORMAL:
-      case META_WINDOW_DIALOG:
-      case META_WINDOW_MODAL_DIALOG:
-      case META_WINDOW_MENU:
-      case META_WINDOW_TOOLBAR:
-      case META_WINDOW_UTILITY:
-        break;
-    }
-
-  return TRUE;
+  return !meta_window_is_skip_taskbar (window);
 }
 
 /*