the string to parse
the length of @buffer (may be -1 if @buffer is nul-terminated)
Return: The merge id for the merged UI. The merge id can be used to unmerge the UI with gtk_ui_manager_remove_ui(). If an error occurred, the return value is 0.
GException on failure.
2.4
Parses a string containing a [UI definition][XML-UI] and merges it with the current contents of @manager. An enclosing <ui> element is added if it is missing.