detector: fix reporting of onsets at negative times
evaluator: disable global onset time limits
evaluator: refactor, improve output

* Refactor code for better modularity.
* Improve calculation of onset accuracy, add precision and recall.
detector: improve onset detection

* Use a better onset detection function
* Reduce window/hop size to 1024/512 samples
vamp plugin: adapt for new onset detection function
detector: use absolute threshold for onset function peaks