Migrate from netcoreapp3.0 to net5.0-windows

[mgnslndh]

This PR aims to migrate Dirkster.AvalonDock from targetting netcoreapp3.0 to net5.0 and net5.0-windows.

Fixes #213

The first step of changing target framework is done and everything seems to work. However there are lots of warnings and I have not yet checked if they are present in netcoreapp3.0. We might want to do more maintenance work.

Let me know what you think when you have tried it out.

[eriove]

Great work. I'm another user of AvalonDock. We are currently using netcoreapp3.0. Would it be possible to keep that target at least for a while? We plan to upgrade but haven't had time yet.

[mgnslndh]

I have no problem with keeping the old net core target platform. Will bring it back in.

[Dirkster99]

Same here, I have no problem with more than 2 target frameworks either (other than increased complexity, more compiling time and larger nuget packages) but we should move out of netcore at some point, maybe in one year, or so?

But right now I am seeing only 2 frameworks eg:

• net5.0-windows;net452 in WinFormsTestApp.csproj
• net5.0-windows;net452 in AvalonDockTest.csproj
• net5.0-windows;net4 in AvalonDock.csproj

Am I looking in the wrong place or are you not yet finished bringing the netcore stuff back in again?

[mgnslndh]

Right, I have not adde back netcoreapp yet. Hopefully i will do it over the weekend.

[mgnslndh]

@eriove @Dirkster99 I have now added netcoreapp3.0 back in.