Extension-Client for Hooks and Triggers

[diovudau]

As wished by @unfa and https://github.com/original-male/non/issues/287 and others

Proposed solution for the requested behaviour: A client that can be added to a session which has slots for various NSM events. You can attach programs to be executed on these events, or save shell code directly in the client to execute.

Request 287 also wants these triggers to influce loading order and speed of nsmd itself, for example delay loading until certain programs ran through ( I can imagine mounting external drives with huge sample libraries, setting up JACK etc.).
While there may exist solutions for this completely external to NSM (why not start the complete session only after the setup is complete?) we do not want to deny this outright but assume there is a valid usecase: This needs further investigation, what can be done with existing OSC messages. For example there is "session is loaded", which the proposed extension-client can delay itself by not answering to the servers announce until is is done setting up.

Read more about extension clients that keep the session managers code-complexity low here: #32