BUG #14303: Restore threshold panel to use two sliders

Revert "BUG #14158: Update threshold panel to use double-ended slider."

This essentially reverts commit 8107ac4f34730f949b8db912247e06aef4ae862c.

Change-Id: I6f0a70340df67ce9320be2b3ce6b9e4b1f4067e1
Merge topic '14303-restore-double-range-widget' into next

2ea9c6e BUG #14303: Restore threshold panel to use two sliders