Core - Fix Broken Build

- DefaultRequestHandler now implements OnDocumentAvailableInMainFrame
- Removed non existent files from project

Upgrade version number to 83.3.120

Update version number to 81.3.100

WPF - Resize Hack for when browser switches from hidden to visible (c…

…efsharp#2779)

- Add 1 to the width/height rather than subtracts like the hack in the previous
versions
- Delay between resizes is now configurable via ResizeHackForIssue2279DelayInMs

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