Missing dispatch_queue_create_with_target and convenience initializer

[VincentDondain]

Missing bindings

We appear to be missing dispatch_queue_create_with_target and a convenience initializer. See https://developer.apple.com/documentation/dispatch/1642205-dispatch_queue_create_with_targe and https://developer.apple.com/documentation/dispatch/dispatchqueue/2300059-init.

Here's our code: https://github.com/xamarin/xamarin-macios/blob/master/src/CoreFoundation/Dispatch.cs

The convenience initializer is being used in a sample ported by QA.

Environment

=== Visual Studio Enterprise 2017 for Mac (Preview) ===

Version 7.6 Preview (7.6 build 2086)
Installation UUID: 00d602ef-271b-479b-9054-0433b5797215
Runtime:
	Mono 5.10.1.57 (2017-12/ea8a24b1bbf) (64-bit)
	GTK+ 2.24.23 (Raleigh theme)
	Xamarin.Mac 4.4.1.178 (master / eeaeb7e6)

	Package version: 510010057

=== NuGet ===

Version: 4.3.1.4445

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	2.1.1
	2.0.5
	2.0.0-preview2-25407-01
	1.1.2
	1.0.5
SDK: /usr/local/share/dotnet/sdk/2.1.301/Sdks
SDK Versions:
	2.1.301
	2.1.4
	2.0.0-preview2-006497
	1.0.4
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.10.1/lib/mono/msbuild/15.0/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.6.3
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Apple Developer Tools ===

Xcode 10.0 (14320.2.4)
Build 10L221o

=== Xamarin.Mac ===

Version: 4.99.2.238 (Visual Studio Enterprise)
Hash: 44816c4ad
Branch: xcode10-metal-b5
Build date: 2018-08-08 15:09:03-0400

=== Xamarin.iOS ===

Version: 11.99.2.118 (Visual Studio Enterprise)
Hash: 44816c4ad
Branch: xcode10-metal-b5
Build date: 2018-08-08 15:09:02-0400

=== Xamarin.Android ===

Version: 9.0.0.17 (Visual Studio Enterprise)
Android SDK: /Users/vidondai/Library/Developer/Xamarin/android-sdk-macosx
	Supported Android versions:
		2.3 (API level 10)
		4.3 (API level 18)
		6.0 (API level 23)
		7.1 (API level 25)
		8.0 (API level 26)
		8.1 (API level 27)

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 27.0.1
SDK Build Tools Version: 27.0.3

Java SDK: /usr
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)

Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Xamarin Inspector ===

Version: 1.4.3
Hash: db27525
Branch: 1.4-release
Build date: Mon, 09 Jul 2018 21:20:18 GMT
Client compatibility: 1

=== Build Information ===

Release ID: 706002086
Git revision: 8499c223e73efdb42f7b5dd7cd53d3477cc31d24
Build date: 2018-07-19 20:45:05+00
Build branch: release-7.6
Xamarin extensions: 4fb0029af6cee260ffb56279b7b17ca51cd4ca18

=== Operating System ===

Mac OS X 10.13.6
Darwin 17.7.0 Darwin Kernel Version 17.7.0
    Thu Jun 21 22:53:14 PDT 2018
    root:xnu-4570.71.2~1/RELEASE_X86_64 x86_64

=== Enabled user installed extensions ===

AddinMaker 1.4.2
NuGet Package Explorer 0.2
Internet of Things (IoT) development (Preview) 7.5