Virtually a complete rewrite of the at_produce plugin:
- Creates at-produce-all and at-produce-selected commands.
- Adds the log node and redraws in the main thread after the separate thread completes. The old code was not thread safe!
- Reports progress periodically using g.es_print.  This is safe because the timer executes in the main thread.
- Rewrote the code using positions, modern iterators, etc.
- Writes the log node as the last top-level node.

Leo build: 20140921062746