Mass generator source/destination field validation

This adds an upfront check to the accept() method for the Mass Generator
to make sure at least one note in the set has both the source and
destination field in order to generate the MP3s.

(cherry picked from commit c103695a2745e205d6c9718d74ed6ad2bfc2bf7e)
Fix usage of thread_id vs _current_id

The former was supposed to be used from the signal, not the value on the
class (which is sometimes the same, but not always).

(cherry picked from commit 67ac8715a952ca7315b6579cef2cd34f1715c537)
Fix for missing arguments in signals

This was happening because the Worker was being deleted before its
signal was passed. This has been fixed now so that the delete is not
permitted until the signal is passed and the callback is executed.

(cherry picked from commit 8c8cee620a1f56300f5aaae12e296cb40ef8a446)