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With M4L you need to implement your feature in a device and add it to your project. My Ableton project template has a bunch of these on my main track. With extensions you use a context menu as the entry point which will hopefully be more lightweight. Hopefully they'll expose more of the object model over time and let us trigger these via keyboard/midi shortcuts.
When you are transcribing a guitar part you have to think about the key, the chords, and how to play the part efficiently. It's a great way to get a better understanding of the neck since there is usually more than one way to play a part.
This is only partially true. Google has started distributing some APIs automatic updates as well via Google Play Services.
http://developer.android.com/google/play-services/index.html
These features are made available on older releases of Android.