Configurable emulation speed and Start core without content

[jfroco]

• Adds configuration for emulation speed percentages: 100% (default), 150%, 200%, 300% and 50%.
• Adds the ability to start the core without content. Taken almost literally from: https://github.com/irisxerno/fuse-libretro/commit/c5c6ddf625b5da8768f82069f2149cc000f22a12
• Adds a configuration option to enable or disable the display of joystick settings at startup.

This should solve the followings issues:

• Start the core without content: https://github.com/libretro/fuse-libretro/issues/141 and https://github.com/libretro/fuse-libretro/issues/134
• Overclock (increase speed): https://github.com/libretro/fuse-libretro/issues/104