Docimport: verbose

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Main window: doc import: Doc import time is fixed (never updated according to the last import) --> better stop the progress bar at 0.75

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Fix doc import:
- Typo in backend/pdf/doc.py
- DocSearch: index_page(): update the whoosh searcher object after indexation + if the page comes from an unknown doc: Add the doc to the internal list

Signed-off-by: Jerome Flesch <jflesch@gmail.com>