WinForms - Update xml doc `Dock = DockStyle.Fill` is the default

Make it clear we are setting the default to DockStyle.Fill, this may not be expected
by some. It has been that way for 5+ years, so changing the default would be difficult
and unexpected, instead we make it clearer this is the case.

Resolves cefsharp#3019

xml doc improvements

Add cefsharp/79 branch details and update build numbers

- Update appveyor.yml/build.ps1 version numbers to 79.1.310
- Created release branch in README.md

Branch to follow shortly

Fixed crash accessing js objects dictionary concurrently

Add a ".NETCoreApp3.0" dependency to the .nuspec files. (cefsharp#2894)

This ensures the transitive dependencies (CefSharp.Common) are correctly resolved in .NET Core 3.0 projects.

Contributes to cefsharp#2796

ICookieManager - Update xml doc

Updated to reflect upstream changes see https://magpcss.org/ceforum/apidocs3/projects/(default)/CefCookieManager.html

Resolves cefsharp#2855

WcfEnabledSubProcess - OnBrowserDestroyed add null checks and log msg…

… if WCF was null

It's likely the WCF host didn't start and some break points should be added in ManagedCefBrowserAdapter::InitializeBrowserProcessServiceHost
to catch the actual exception. We're not logging as it causes too many false positives as it's expected there will be errors
when browser is created then rapidly Disposed.

Resolves cefsharp#2839

UrlRequestFlags - Update with new values

https://bitbucket.org/chromiumembedded/cef/commits/cd5995a440302e0ef5a97a000faa922149ffb177?at=3683#Linclude/internal/cef_types.hT1281

Revert "build.ps1 - Upgrade to support new version scheme"

This reverts commit 1f39e42.

Upgrade to CEF 3.3578.1870.gc974488 / Chromium 71.0.3578.98