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

Updating Threshold panel (i.e. pqDoubleRangeSliderPropertyWidget) to use double ended
slider instead of two separate sliders. This overcomes wierd sync and update issues
that were common with Threshold panel esp. on Macs.

Change-Id: I2afad95c7d582a547c03974da9830554fe320f7c
Merge topic '14158_update_threshold_panel' into next

8107ac4 BUG #14158: Update threshold panel to use double-ended slider.