Dragging slider bar in properties panel creates too many undo events

When you drag a slider bar for one of the properties in the properties panel that is automatically updated, it creates an undo event for every incremental update provided by the GUI, which needlessly fills the undo stack. To replicate, do the following:

1. Create a Sphere source. Apply.
2. In the properties panel, slowly drag the slider for opacity.
3. Try undo. Note that each undo is a small and worthless amount.

One possible fix for this is to ignore incremental updates and only respond when the user releases the mouse from the slider. This is probably the easiest fix plus it would prevent problems when an update takes longer than a few milliseconds.