Using ETEXT_DESC for status update

Hi,
for my project I needed the recognizing status. Within the headers I found  ETEXT_DESC which exactly does what I need. You just need to pass it's pointer to `_tesseract->Recognize()`.
In order to update the value I created a thread which periodically checks the progress field of ETEXT_DESC for changes.
Maybe you'd like to merge into your master branch after testing. For me it works quiet well.