Add net5.0-windows

[inforithmics]

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

added net5.0-windows target and Sample

⤵️ What is the current behavior?

not possibility to target net5.0-windows

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

target net5.0-windows

💥 Does this PR introduce a breaking change?

no

🐛 Recommendations for testing

start demo.windows sample

📝 Links to relevant issues/docs

the net5-windows target uses xamarin.forms 5 because 4.5 has no net5-windows target.
the other targets are still on Xamarin.Forms 4.5.

Another possibility would be to target all to Xamarin.forms 5.0 but then monodroid9.0 support would have to be dropped.
or still target Xamarin.forms 4.5

Another possibility would be to Fix the .NetCoreApp 3.1 Target to target Xamarin.Forms Wpf

🤔 Checklist before submitting

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

Further fixes:

I further fixed the uwp compilation error and tizen although compiles now.

[LuckyDucko]

hey @martijn00 I am a bit out of my depth concerning windows development.

This PR strikes me as all good, as it is just pushing forward with .net 5.0 for the new Windows WPF target.

[martijn00]

I'll have to look at this later. I can't get the whole .net5 update to work with the nuget package. It might be fixed in a later VS update.