Add MidiTest sample

Creating branch until required API fixes are published
Merge branch 'MidiTest' into monotouch-7.2.1
Fix bug 18516
Merge branch 'master' of github.com:xamarin/monotouch-samples into monotouch-7.2.1
[MidiTest] Merge changes from Miguel

Show notes being played usign async, remove redundant and useless stub code
Merge pull request #36 from xamarin/monotouch-7.2.1

Merge monotouch-7.2.1 branch into master