plugin base: let plugins decide if they should be sorted last

Note that even if a plugin says it wants to be sorted last, the generic plugin
will always be the last one. Also, there is no order guaranteed between two
plugins that request to be sorted last.

src/mm-plugin-base.h

@@ -106,7 +106,8 @@ void mm_plugin_base_supports_task_add_custom_init_command (MMPluginBaseSupportsT
 #define MM_IS_PLUBIN_BASE_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass),  MM_TYPE_PLUGIN_BASE))
 #define MM_PLUGIN_BASE_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj),  MM_TYPE_PLUGIN_BASE, MMPluginBaseClass))
 
-#define MM_PLUGIN_BASE_NAME "name"
+#define MM_PLUGIN_BASE_NAME      "name"
+#define MM_PLUGIN_BASE_SORT_LAST "sort-last"
 
 typedef struct _MMPluginBase MMPluginBase;
 typedef struct _MMPluginBaseClass MMPluginBaseClass;