Fix #132 & Update .NET versions

[BornToBeRoot]

• This will fix #132 (Tab is locked and cannot be selected/dragged in some cases)
• Use latest .NET versions:
  • .NET 6
  • .NET 3.1
  • .NET-Framework 4.8
• Some code cleanup

[BornToBeRoot]

You need to update the AppVeyor config to use .NET 6 😄