initial support for custom tape loaders

working:
  tzx turbo tape blocks
  tzx pure tone tape blocks
  tzx pulse sequence tape blocks
  tzx pure data tape blocks
  auto select tzx loading mode depending on file contents

not working/implemented:
  tzx direct recording tape blocks (havent found any files using these)
  some speedlock 4 games dont load reliably
  manually switch tzx loading mode (for games that use custom load routines with standard tzx tape blocks eg hewson)
  sound (no audible tape tones)
  start/stop tape control
  accelerated loading
  select tape block controls (for multi load games)