[Cin] Gain audio plugin now has Textbox and Clear button

A User contributed improvement to the Gain audio plugin has been checked into GIT. It is quite a few more lines of code to add a Textbox and a Clear/Reset button so the work is very much appreciated. The Textbox makes it so much easier to get a precise number plugged in rather than trying to just get the slider bar positioned exactly right and, of course, the Clear button gives us a fresh start. Always thankful for User contributions with these types of improvements! .