Port to .NET Core

[nikeee]

.NET Core 3.0 will be releasing with Windows Forms support.

[nikeee]

This PR has landed:
dotnet/winforms#57

We may remove our custom solution in favour of this.

[nikeee]

MEF 1.0 might be an issue. But 2.0 it seems to be supported somehow: https://blog.softwarepotential.com/porting-to-net-standard-2-0-part-2-porting-mef-1-0-to-mef-2-0-on-net-core/

[nikeee]

https://devblogs.microsoft.com/dotnet/how-to-port-desktop-applications-to-net-core-3-0/

[nikeee]

Porting the main application is blocked due to Windows Forms not being available with VB.NET:
dotnet/runtime#26866

There is no project template. Also WindowsFormsApplicationBase is not implemented.

[nikeee]

WindowsFormsApplicationBase is now available in .NET Core / Windows Forms.

However the Single Instance Feature has not been ported. We have to wait until this issue is resolved: dotnet/winforms#2056

[nikeee]

We will be able to remove some fallbacks that are currently provided by the Windows API Code Pack (see #15) as soon as we have the Winforms that comes with .NET Core.

This is a checklist for the migration:

• Vista-Like FolderBrowserDialog
• VistaLike Common File Dialog
• TextBoxes can now contain prompts/placeholders

[nikeee]

dotnet/winforms#2056 is now resolved. We should now be able to port HS to .NET Core / .NET 5.

[nikeee]

Some Controls we use do not exist any more:
https://docs.microsoft.com/de-de/dotnet/core/compatibility/winforms

[nikeee]

This basically works. It now depends on .NET 5 being released (currently in preview only).

Some parts are still to do in #34.