preset: add gst_preset_is_editable()

Add a function to check if the preset iface implementation is editable and
document this from the implementers perspective.
API: gst_preset_is_editable()

gst/gstpreset.c

@@ -34,6 +34,9 @@
  * native preset format of those wrapped plugins.
  * One method that is useful to be overridden is gst_preset_get_property_names().
  * With that one can control which properties are saved and in which order.
+ * When implementing support for read-only presets, one should set the vmethods
+ * for gst_preset_save_preset() and gst_preset_delete_preset() to %NULL.
+ * Applications can use gst_preset_is_editable() to check for that.
  *
  * The default implementation supports presets located in a system directory, 
  * application specific directory and in the users home directory. When getting
@@ -1148,6 +1151,24 @@ gst_preset_get_app_dir (void)
   return preset_app_dir;
 }
 
+/**
+ * gst_preset_is_editable:
+ * @preset: a #GObject that implements #GstPreset
+ *
+ * Check if one can add new presets, change existing ones and remove presets.
+ *
+ * Returns: %TRUE if presets are editable or %FALSE if they are static
+ *
+ * Since: 1.6
+ */
+gboolean
+gst_preset_is_editable (GstPreset * preset)
+{
+  GstPresetInterface *iface = GST_PRESET_GET_INTERFACE (preset);
+
+  return iface->save_preset && iface->delete_preset;
+}
+
 /* class internals */
 
 static void

gst/gstpreset.h

@@ -97,6 +97,7 @@ gboolean     gst_preset_get_meta           (GstPreset *preset, const gchar *name
 gboolean     gst_preset_set_app_dir        (const gchar *app_dir);
 const gchar *gst_preset_get_app_dir        (void);
 
+gboolean     gst_preset_is_editable        (GstPreset *preset);
 G_END_DECLS
 
 #endif /* __GST_PRESET_H__ */