Remove implicit deactivation of Shutter Auto mode

Story

Currently, manually opening / closing the shutter implicitly disables the auto mode for shutter handling in scans, which is quite annoying: If you manually open and close the shutter (e.g. during alignments) and forget to activate auto mode afterwards, your next scan will be garbage due to a closed shutter. The same is true, if you - for any reason - need to manually close the shutter during a scan and forget to re-activate auto mode before the next scan.

Requirements

• Pushing the OPEN resp. CLOSE button in the ShutterWidget shall not change the shutter's Auto mode setting. The same applies to calling the open_shutter or close_shutter methods.
• Disabling Auto mode shall never change the shutter state.
• Enabling Auto mode shall not have an immediate effect, but on the next start or stop of a scan.

This means for activated Auto mode:

• If I open the shutter manually before starting a scan, it is automatically closed at the end of the scan (staying open all time until then).
• If I close the shutter manually during a scan, it is automatically opened at the begin of the next scan (staying closed all time until then).
• If I deactivate Auto mode during the scan, the shutter stays open beyond the end of the scan.

This means for deactivated Auto mode:

• If I activate Auto mode during a scan with open shutter, it is automatically closed at the end of the scan.
• If I activate Auto mode while no scan is running, an open shutter stays open.