Various UI improvements

[michaeltyson]

Hi! A few proposed changes here:

1. A trivial one first: tweaked the appearance of the saved game list, so that it has a transparent background, and selected items have a semitransparent white background with rounded border,
2. Updated joystick library to latest from bradhowes/Joystick,
3. Implemented dual joysticks with standard scheme - left controls forward and sideways motion, right controls pitch and yaw,
4. Adjusted joystick control curve from linear to power curve for more natural control,
5. Revised joystick to use relative, rather than absolute position, for control, meaning it's less critical where the first touch occurs,
6. Added tap on right joystick to fire,
7. Tweaked control system in cl_input.c for smoother movement,
8. Addressed an iOS quirk that caused random delays to the touch end event, causing mayhem