Only include WPF on .NET Framework project

[kvpt]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

The WPF platform files are included by this condition :

$(TargetFramework.StartsWith('net4')) And '$(OS)' == 'Windows_NT'

but the WPF framework included by the UseWPF property is included by the condition :

'$(OS)' == 'Windows_NT'

Use the same condition to not include Wpf when not in NET Framework project.

:arrow_heading_down: What is the current behavior?

When creating a test project targeting net5.0/net6.0 that reference a library that using this library, the build fail because the WPF FrameworkReferences is required, this force to use net5.0-windows or net6.0-windows. The build fail with error NETSDK1136.

:new: What is the new behavior (if this is a feature change)?

WPF is now only required on .NET Framework project.

:boom: Does this PR introduce a breaking change?

No because the WPF framework was required but the platform files was not included on non .NET Framework project.

:bug: Recommendations for testing

Reference the library in a project targeting net5.0/net6.0, the build should not fail with error NETSDK1136.

:memo: Links to relevant issues/docs

Issue #568

:thinking: Checklist before submitting

• [X] All projects build
• [X] Follows style guide lines
• [X] Relevant documentation was updated
• [X] Rebased onto current develop