Fix slider debouncing, fix slider steps, fix slider clear

[dauglyon]

Fixes three items from the bug punchlist:

• closest_placement_af and possibly other sliders show errors after sliding and cannot be recovered
  • fixed by setting step
• Filter slider handle does not move until have mouse-up
  • fixed by isolating slider state from form/filter state
• Removing filter doesn’t reset the filter UI
  • fixed by adding an effect to filter controls