Frontend: Scans: Fix support of scanners with weird source naming convention

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Cleanup: Take advantage of the cleanup in the pyinsane API to clean up here as well.

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Main window: Fix: Disable correctly the label list when no document is selected

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