Solves #8 issue.

[jmbau]

Features:

• Supports full mapping for up to 3 joysticks with up to 8 axes, 16 buttons and 2 POV hats per joystick.
• Supports movement inversion for axes and POV hats.
• Supports joystick hot plug on joystick test page (not in game).
• Joysticks connection order now don't matter (each joystick unit is detected by its SDL "Globally Unique ID").
• Supports "move axis to select" for axes and hats mapping, as it was already possible for buttons.
• Point Of View control can now be mapped on two analog axes, not only on a hat.
• Air brakes on/off state now works properly when mapped to a joystick button (was jiggling).
• Wheel brakes now works as hereunder when mapped to a joystick button:
  • Push button to apply wheel brakes, release button to release wheel brakes (no modification here).
  • Push button and press <'shift'> key then release button before release <'shift'> key to activate parking brake.
  • When in parking brake state, click (push then release) button to release brakes.
• Wheel brakes works as hereunder when mapped to one or two analog axes:
  • Move axis (brake pedal) to apply braking, release to release braking.
  • When braking power is 100%, press <'shift'> key then release braking before release <'shift'> key to activate parking brake.
  • When in parking brake state, move axis (pedal) to at least 1% then release it to release parking brake.
• Adds a [Keyboard...] button in controller options menu (see known bugs).

Known bugs:

• When an old sar.ini configuration file is read, mapping is lost. There is a warning about that, but it is only visible from command line.
• In poor resolutions (640x480/VGA and 720x400), test page is readable with max 2 joysticks (not 3) and mapping page is really not beautifull but works. From SVGA (800x600) and upper resolutions, all is okay.
• Wheel brakes <'shift'> key detection don't work very good on joystick test page but works perfectly while in game.
• New [Keyboard...] button in controller options menu don't work (disabled/grayed-out). The reason of this button is that on my keyboard wheel brakes / parking brakes command is really very difficult to use because of <'period'> (.) and <'greater than'> (>) keys positions are radically different on my AZERTY keyboard than on a QWERTY one. Because wheel brake action works now fine when mapped on a joystick button or analog pedal(s), I didn't work on that yet.
• If Point Of View control is mapped on two analog axes, it will works fine only if axes values are near to zero (+/-1% dead bandwidth) when axes are at their rest position. It not, POV will drift.

SVGA resolution window captures: