Improve performance profiling by modifying qx.event.Idle

Changed to only create a Timer while there are listeners for the interval event and removes Timer after last listener released. This can reduce the number of profiled function calls from 9,000 in 10 seconds to zero when Idle is not required, massively simplifying the profile process.

Relates to http://bugs.qooxdoo.org/show_bug.cgi?id=6115