add net5 target framework

[6bee]

This PR is about adding target framework net5.0 and to generate nuget package on release build.

[WrongBit]

What a reason to target raw, clumsy .NET 5 if it's not even ready? No WinForms, no WPF... WHERE you suppose to run FCTB?? Not saying you break project file in favor of 3 developers who use .NET 5 (against thousands who sit on FW 4.8).

[Ericvf]

Meanwhile I have no issues with creating a new .NET 5 WinForms project and installing FCTB from Nuget.
Aside from the .NET targeting warning it works as expected.

[6bee]

Using FCTB.2.16.24 package in a winforms project targetig net5.0-windows gives me the following error/warning:

NU1701 Package 'FCTB 2.16.24' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8' instead of the project target framework 'net5.0-windows7.0'. This package may not be fully compatible with your project.

It might be worth noting the package doesn't even follow the guideline to specify a TFM for the dll in lib folder.

Apparently no one dares to update this project though!? However, having created this PR I see my duty fulfilled in regards of the oh-so-awesome LGPLv3 license terms...

[SjaaLom]

Would be nice to get rid of the warning when targeting net5. Can someone please approve this PR?