edit pre-emphasis value in text box

[camsinfiniteram]

Previously, only the slider could edit the pre-emphasis value. This change allows you to edit the value by typing it into the text box, then pressing "ENTER" on your keyboard. The slider and textbox should now be synced to maintain the desired value, regardless of which method is used to adjust it.

Next, see if there are any potential performance issues.

Tested:

• Alternating between the slider and text box to adjust pre-emphasis value
• Using just one for multiple increments
• Switching between positive and negative values to see if both worked

[cwnaber]

Cam and I tested this some more this morning and it seems all good.