[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.