Update changelog

Migrate to ICSharpCode.ILSpyX 10.0 Preview 2

Also upgrade some more dependencies

Remove commands from command palette needing a context

... like a node selection in some tree panel. They would fail anyway and
don't make much sense.

Update changelog for 0.21 release

Update changelog for 0.20 release with wiki link

Merge pull request #438 from marcinjahn/patch-1

Prepare 0.18 release

Preparation for 0.17 release