WHDLoad changes (button overrides)

Created by: sonninnos

• New WHDLoad boot overrides (keyboard input removed):
  • Red for Selector
  • Blue for Config
  • Red+Blue for Readme+MkCustom
  • LMB for Splash
  • RMB for Config+Splash
• Renamed "WHDLoad.prefs Options" to "WHDLoad Splash Screen Options" + clarified options
• Updated Readme Closes #258 (closed)

Bonus:

• 80MB option for HDF creation
• Fixed directory launching breakage because of a file_exists check