Set the stage to pack and push 31.0.0-pre1 CefSharp NuGet for #137

- Bump version series from 3.<something> to the new 31.0.0-pre1. Note with this we don't even have a clash with unofficial version id's pushed to staging.nuget.org :-)
- Change `cef.redist` dependency version indicator to allow auto-update or install of latest & greatest in the 3.1650.1562 series
Let's add a <licenseUrl> to packages
Cefsharp.Common.targets: cleanup and don't hijack AfterBuild
Merge pull request #314 from jornh/prepare-to-pack-31.0.0-pre1

For #137: Set the stage to pack and push 31.0.0-pre1 CefSharp NuGets