Lots of internal changes to the game engine

Now using SDL 2.0.7 and SDL_mixer 2.0.2

New MOD capabilities added for creating community content

New menu for nerds, including technical game-engine stats

Removed analogue joystick option (it never worked very well)

User levels are now saved in user directory (with save game)

Improved highscore table visuals

Fixed touch controls in Windows version

Various visual improvements to menus

More sound effects added