MyGet Key Update

Update to using a limited access expiry key from my account (amaitland)

Nupkg Restructure

Resolves cefsharp#2156

Nuget packages updated to use ItemGroup None instead of Copy target
Nuget props restructure to support Win32 target
x86 and Win32 targets are exactly the same in content, they differ in name only, so switched to using an <Otherwise/> block
Remove AnyCpu <When/> option from props files and consolidate with x86 and Win32. Set Private to false and rely on the
<None/> entires to copy the files to the output folder

Ideally would move the OffScreen, WinForms and WPF targets file content to the props (as they're actually props now), unfortunately
we need guaranteed ordering and Nuget 2.8.x doesn't provide that, if we leave them as .targets it'll happen late enough to make sure
the cef.redist list of files is provided.
NuGet/Home#4229 (comment)

README.md - Add cefsharp/63 branch details

(branch to follow)

appveyor.yml: Update api_key

The previous key belonged to Alex Maitland, who has revoked it. Updating to use my own key instead.

Upgrade to CEF 3.2987.1601

Add 57.0.0 branch details (branch to follow)

Upgrade to 3.2883.1552

Add isMainFrame to IRequestContextHandler.OnBeforePluginLoad (CEF API changed)

Add reference to the cefsharp/55 branch (will be created shortly)

Upgrade to CEF 3.2785.1486

Only set keyboard handler in Winforms example of using multiThreadedM…

…essageLoopEnabled