cef.sdk.nuspec & friends used for cef.sdk.3.1650.1562-pre2 NuGet

This is what was used to build the cef.sdk.3.1650.1562-pre2 NuGet currently on staging.nuget.org
It comes with extra DRYness which CefSharp.Core now relies on since the SHA a7590937 commit https://github.com/jornh/CefSharp/commit/a7590937ccda3c4506827a91d81bd8093b672ec4
    <CefSdkVer>cef.sdk.3.1650.1562-pre2</CefSdkVer> in cef.sdk.props does that

Note: Contents in the CEF/ folder is currently still a manual cherrypick from building upstream CEF
Resolution of this is tracked in https://github.com/cefsharp/cef-binary/issues/5