This PR implements the following features:

• Added strong name signing as requested in multiple issues
• Move build "engine" from MyGet.org (Batch script) to Github.com (yml-based actions)
• Adds .NET 5.0 as new, additional target

Added NuGet readme, replaced PackageIconUrl by PackageIcon, etc.

Removes MyGet build script and introduces Github Actions as new "build engine".

Codecov Report

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@@            Coverage Diff            @@
##             master     #331   +/-   ##
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  Coverage          ?   61.95%           
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  Files             ?       17           
  Lines             ?     2560           
  Branches          ?      486           
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  Hits              ?     1586           
  Misses            ?      906           
  Partials          ?       68           

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