feat: expose „show at login“ setting to determine if scan screen should be shown

[hansemannn]

Hey there!

This pull adds a new setting to toggle if the scan window (ScanWindowController) should be shown on login. In addition to the "start at login" functionality, this one is useful for users who do not want to see the scan window on every start, which was my personal motivation on this as well.

If you have further questions, I'd be happy to answer those!

Note 1: If okay, I could also move the project to the latest Xcode 10.2 settings to fix / silent some project warnings Note 2: If interested, I could offer German translations for this project as a continuous effort.