File corruption fix, undo feature WIP

Potentially fixed the corruption that causes stderr output to end up in
processed files somehow, by not outputting anything to stderr anymore.
This bug seemed to only happen for certain long pathnames with spaces
and other undetermined characteristics.

Added methods to call MP3Gain's undo feature, not yet exposed in the UI.
Added Prevent Clipping option, hooked up Undo

- Added a new checkbox next to the desired volume called "Prevent
Clipping". If checked and you're increasing the volume of an MP3, but
the desired volume would result in clipping the highest optimal value
before clipping occurs is used instead.
- Undo option added to UI that removes volume gain added to a file,
restoring the original volume.
Increase version to 1.1

Increment version number to 1.1 in preparation for another release