The progress reporting function returns a boolean. The returned
value is never used by the tesseract and its meaing is not
documented, which renders the value meaningless. Still, lack of
return should not be premitted.
The progress_callback field in the ETEXT_DESC monitor type does not
take any 'context' parameter, which may make implementing callback
functions difficult and may require use of global variables.
The new function receives the ETEXT_DESC pointer as an argument.
This makes it possible to share the cancel_this field as context
carrier if required.
The change is backwards-compatible: the old pointer remains as a
member of the class, and the default value for the new pointer is
a function calling the classic progress notifier. This way the code
unaware of the new member will continue to work as before.
