Only apply quality slider when released

[rotbus19]

Applies to Export dialog with 'Automatic preview refresh' checked

• Current behavior: if you drag the quality slider in the Export dialog, the quality is applied without releasing the slider, thereby locking up the UI until that quality setting is applied. If you are working with a very large image, this makes applying quality settings very time consuming and frustrating.
• Proposed behavior: do not apply quality setting until the slider is released