Add option to re-label previous sample

[scottveirs]

On rare occasion, often while trying to label as fast as possible, I select a label just prior to the end of the clip but hear a potential signal at the same moment. In those cases, I may have mis-labeled the sample, so it would be nice to be able to go back, listen to the previous sample again (in it's entirety) and then have the option of changing the label. Maybe the UI for this feature could be located near the "Skip" button?