Busy pointer is too busy

[soininen]

The busy mouse pointer we set using the busy_effect decorator often blinks like every millisecond, at least on Windows, which is disturbing. It could be either due to nested calls to busy_effect or perhaps we call busy_effect in a tight loop. Either way, we should make sure the pointer stays busy for the entire duration of the action.

[ptsavol]

I noticed a while back that the busy_effect decorator don't work if the work happens in another thread. One sure way to make it work is to change the cursor only in the main thread.