Add support for sample tempo related time stretching

Currently, time stretching for loops can only be applied, when you set the correct beat sync value and when the sample 100% match the beat sync value. It would be nice, when we can set a sample tempo, so the time stretcher can use this instead of the beat sync value.

It can be a new mode in the beat sync selector as SmpTmp (Sample tempo):
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WAVE/RIFF have a tempo chunk (TEMPOMAP), which can be read out. It could also use the file name or sample length to determine the tempo. Sononym does detect the tempo already.

A sample tempo can be displayed here:
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And the tempo could be adjusted here as new option:
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Btw.: I build me a little script to extend the sample length, so it matches one of the beat sync values. Works pretty fine for me, but i think this could be solved natively:
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