I have used UP/DOWN to distinguish between emit during press = down and emit after release up. The wheel should be disabled in a emit on release slider, so it has to be down. You can recall the value by clicking. 

This discussion shows that this way of connection is hard to understand from the source. So please decide which way want to go. For me all of these are reasonable:  
1. This solution, it could be self explaining for one who don't likes to read the manual 
2. Introduce a Slider Widget Option "EmitWhileMoving" or something 
3. Make a dedicated widget class for it.