[python] add xbmc.abortEvent

I intend this as a simple replacement of the boolean property. It allows add-ons to use `xbmc.abortEvent.isSet()` instead, and most importantly `xbmc.abortEvent.wait()`instead of periodically checking.

Missing:
- Documentation: surely this is currently done by magic, so I'm hoping at least the abortEvent *attribute* should be documented.
- Deprecation of `abortRequested`: need elaborate hacks in initialization script to do this.
unrelated changes?