Adds a UI element to the current contents of manager.
If type is GTK_UI_MANAGER_AUTO, GTK+ inserts a menuitem, toolitem or separator if such an element can be inserted at the place determined by path. Otherwise type must indicate an element that can be inserted at the place determined by path.
If path points to a menuitem or toolitem, the new element will be inserted before or after this item, depending on top.
the merge id for the merged UI, see gtk_ui_manager_new_merge_id()
the name for the added UI element
the name of the action to be proxied, or NULL to add a separator
the type of UI element to add.
if TRUE, the UI element is added before its siblings, otherwise it is added after its siblings.