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.
The merge id for the merged UI. The merge id can be used
to unmerge the UI with gtk.uimanager.UIManager.removeUi. If an error occurred,
the return value is 0.
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.