Implement `PLAYBACKRATE` in modules

[Gumball2415]

PLAYBACKRATE is a setting in FT 0.5.0b which can directly dictate the NSF play speed instead of a given engine tick rate in Hertz.

There are three types of playback rate settings:

• Default NSF rate, which is 16666 (60.002 Hz) for NTSC and 20000 (50 Hz) for PAL
• Custom NSF rate, which sets both the NTSC and PAL playback rate to a custom rate.
  • See below for more details
• NES video rate, which is 16666 (60.002 Hz) for NTSC and 20000 (50 Hz) for PAL.
  • Ideally, it would be 16639 (60.01 Hz) for NTSC and 19997 (50.008) for PAL.

There are three methods to set the Custom NSF rate in the tracker:

• Directly through a spinbox, in microseconds
• In Hertz, which will then modify the spinbox value
• In beats per minute, which will then modify the spinbox value

The tracker also notes that only engine speeds at 60Hz below are guaranteed to work when exporting to NSF.