(cherry picked from commit f64e5325)

(cherry picked from commit 75a1facb)

Also move msbuild.sdk.extras in global.json in solution root to manage its version more easily

This commit contains a LibVLCSharp integration with GTK with optional
Xamarin.Forms support. It works on Linux and Windows only
(see https://github.com/mono/gtk-sharp/issues/257 as to why).

(cherry picked from commit baf5d7c0bf505f3b797fc13f6a08e575db833add)

Given that the WINDOWS constant is meaningless (Linux mono builds as
a .net framework too, and have WINDOWS set), and that .net core builds
running on windows should execute the same actions as .net fx,
I introduced a DESKTOP flag, for standard, core and framework.

Consolidated packages versions

Updated MSBuild.Sdk.Extras

removed useless unsafe blocks

(cherry picked from commit 4ddf9bdf083c71e2c64874c20c0eccc9fbd1cf2a)

* Add Forms.Android setup

* Add Forms.iOS setup

* Add Xamarin.Forms support to README

* implicitly call attach/detach

* bump TargetFrameworkVersion to 8.1