Don't interrupt scanning if the device is busy.
After scan is finished, the scanner needs few of seconds to prepare for next scan. During that moment the device is in Sane.Status.DEVICE_BUSY state. Previously if the user press "Scan" button during that state, the message: "Unable to start scan" was be displayed.
This patch fix that. After pressing Scan button in DEVICE_BUSY state, the application will keep trying start next scan. If the user would like to interrupt this loop, he could press "Stop" button.
(cherry picked from commit 60e8ce7a)