Massive performance optimization. Cache the transferChanged and transferGroupChanged event directly on the TransferTreeModel class by starting a timer when an event happens and committing all the gui changes of all the transfers and groups after 200 msec. Note that at this point the timer is stopped and a new one is started only if another event occurs.

This drops the cpu usage with the multisegments plugin from ~80% to ~5% on my machine. It's a good result, but not enough..