Change TFMs of ILSpy/Decompiler Projects

[christophwille]

Switch ILSpy project to 4.6.2 (from 4.6).

Change TFMs of Decompiler from net45;net46;netstandard2.0 to netstandard2.0 only. net46 was for earlier days of Roslyn, now we can target netstandard2.0 for current releases. net45 was only for a single dependent project.

[christophwille]

A note when opening the ilspy.zip file for the first time: please see https://stackoverflow.com/questions/47184230/large-number-of-assemblies-in-net461-output-folder-when-building-net-framework

We intentionally only stepped up to 4.6.2 (and not 4.7.1 https://en.wikipedia.org/wiki/.NET_Framework_version_history#.NET_Framework_4.7.1 which adds full netstandard2.0 support - released mid-October 2017) to be cognizant of machines with older frameworks only. The zip file size is only minimally bigger.

The addin is not affected because it runs only in VS greater/equal 2017 (and VS 2017 installs 4.7.x)