Rename source files ("DONE " prefix) after successfull conversion

I want to be able to identify processed files easily as "processed" on the source disc (I often have several files from DVB-T stream and I am not doing conversion of all files at once). Also sometimes I want to keep archived full-quality files (the BBC Earth files are great).

This enhancement prepends "DONE " string to processed filename. This is done only if the cb_remove isn't set.

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Possible enhancements:
- define prefix string
- add post-process checkbox and group "remove file" and "rename file" as radio button alternatives:
<pre>
[x] post processing
    ( ) Delete input file after conversion
    (o) Rename input file after conversion
</pre>
with disabling both radio buttons when the post-processiong is not enabled.