Restoring support for dynamically closing/openinig midi callbacks

* autohotkey doesn't have an array property that allows you to see the number of members, only the highest index (which isn't the same thing)
* so we have to keep a counter of the number of open midi handles ourselves
* the original on the fly callback open/close functionality works again
Bugfixes from previous refactoring

* we now keep the last global event and the last event per handle, to help make it easier to read events from all devices
* eventually we will do more with this, so you can query events for just one device
* fixed problem with midi close routines caused by autohotkey issue with iterating over an array while removing members from it
Updating code snippet in readme to reflex current codebase