Auto-discard empty recordings

After recording is stopped, automatically delete all newly created empty items.

Empty being defined as:

- midi:. no midi data recorded, or less than X number of midi events (for those broken pitch wheels that fire off data at random, like mine)

- audio: no level recorded above a given threshold (to disregard noise)


original discussion here: http://forum.cockos.com/showthread.php?t=37447

(this feature should be an option in the preferences)