Improve DTMF detection

[fwojcik]

This PR contains a few improvements to detecting DTMF signals:

• Make FFT inputs look more like periodic inputs, to reduce low-frequency noise
• Slide input window by less than 100% (default to 50%)
• Consider more frequencies than just highest amplitude when matching
• Don't consider high frequencies to be a match if they are too weak (default to minimum of 1/3 of low frequency amplitude)

It also has some minor code cleanups.