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

[Phyllis Smith]

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! .
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