add_ui_from_resource
Description:
Parses a resource file containing a UI definition and merges it with the current contents of this.
Parameters:
| this | a UIManager object | 
| resource_path | the resource path of the file to parse | 
Returns:
| The merge id for the merged UI. The merge id can be used to unmerge the UI with remove_ui. If an error occurred, the return value is 0. |