[Mac] Upgrade Protobuild, Allow MacOS to target newer Xamarin APIs, etc.

This PR addresses multiple issues at once because they're all dependent on one another:

* Upgrades Protobuild: Throughout the course of this PR I had to make changes to Protobuild to support the newer APIs and fix various issues across platforms, so that is updated here.
* Remove old XSLTs: With the Windows Universal and tvOS platforms upstreamed into Protobuild, there's no need for MonoGame to keep it's own copies of the XSLT files any more.  This PR removes them in favour of the built-in ones.  This will assist with MonoGame#4218.
* Mac now supports the Xamarin.Mac Unified API: It supports both the MonoMac API, XamMac API and newer Xamarin.Mac API (unified).  The older APIs are supported by #ifdef'ing namespaces with the `PLATFORM_MACOS_LEGACY` define.  This resolves MonoGame#4275, but should also fix bugs like MonoGame#4278.
* WindowsUAP is now WindowsUniversal: This brings the MonoGame platform name inline with the platform name being used in upstream.  See https://github.com/hach-que/Protobuild/pull/130 for the name change reasoning.
* WindowsUniversal package resolution occurs outside MSBuild: Until the build server is updated, this is necessary to build against an older version of the Windows Universal SDK.

.gitignore

@@ -40,8 +40,8 @@ obj/
 Ankh.NoLoad
 .vs/
 project.lock.json
-/MonoGame.Framework/MonoGame.Framework.Net.WindowsUAP.project.lock.json
-/MonoGame.Framework/MonoGame.Framework.WindowsUAP.project.lock.json
+/MonoGame.Framework/MonoGame.Framework.Net.WindowsUniversal.project.lock.json
+/MonoGame.Framework/MonoGame.Framework.WindowsUniversal.project.lock.json
 artifacts/
 
 #Tooling