Major input handling improvements (#28):

- Support joystick hotplugging
- Support axes as buttons (means xbox360 shoulder triggers can be used as buttons)
- Show pretty joystick axis & button names if possible
- Tabify input actions UI
- Add 'mouse' column to input actions UI
- Allow to configure form hotkeys
- Allow ALL keys, get rid of internal key remapping
- SDL2: Use scancodes instead of keycodes since they are layout independent
- Allow extra mouse buttons (if present)
- Remove "lmbd" & "lmbu" actions in favor of "PrimaryAction" & "SecondaryAction"
  Makes the configuration less redundant and doesn't send each action twice,
  which happened if both were set to the same key.
- Fix Regressions from prev commits (menu not opening on Esc)

Still has a few minor bugs/issues that need to be fixed,
but pushing this now before the commit gets too large again.

BBGE/ActionMapper.h

@@ -45,15 +45,18 @@ struct ActionData
 enum ActionButtonType
 {
 	// must start at > 512 (SDL scancodes end there)
-	MOUSE_BUTTON_LEFT	=  999,
-	MOUSE_BUTTON_RIGHT	= 1000,
-	MOUSE_BUTTON_MIDDLE	= 1001,
+	MOUSE_BUTTON_LEFT	=  1000,
+	MOUSE_BUTTON_RIGHT	= 1001,
+	MOUSE_BUTTON_MIDDLE	= 1002,
+	MOUSE_BUTTON_EXTRA_START = 1003,
 
 	JOY_BUTTON_0		= 2000,
 	JOY_BUTTON_END = JOY_BUTTON_0 + MAX_JOYSTICK_BTN, // one past end
 
-	JOY_AXIS_0			= 3000,
-	JOY_AXIS_END = JOY_AXIS_0 + MAX_JOYSTICK_AXIS,
+	JOY_AXIS_0_POS			= 3000,
+	JOY_AXIS_0_NEG			= 3100,
+	JOY_AXIS_END_POS = JOY_AXIS_0_POS + MAX_JOYSTICK_AXIS,
+	JOY_AXIS_END_NEG = JOY_AXIS_0_NEG + MAX_JOYSTICK_AXIS,
 };
 
 class ActionMapper