keymap: limit set of left/right swap options

For Shift, Control, Option, and Command assume that you want to shift
on the same side, left-for-left, right-for-right. The Menu key is
available on either the left or right side, and lock keys can be
swapped with any other modifier left or right.

If we have multiple unmapped modifiers we need to do a little extra
work to figure out which of the unmapped modifiers to swap with.

Convert private class member methods to static functions.

Also update copyright info

src/preferences/keymap/KeymapWindow.h

@@ -1,12 +1,13 @@
 /*
- * Copyright 2004-2009 Haiku Inc. All rights reserved.
+ * Copyright 2004-2014 Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  *
  * Authors:
- *		Sandor Vroemisse
- *		Jérôme Duval
  *		Alexandre Deckner, [email protected]
- *		Axel Dörfler, [email protected].
+ *		Axel Dörfler, [email protected]
+ *		Jérôme Duval
+ *		John Scipione, [email protected]
+ *		Sandor Vroemisse
  */
 #ifndef KEYMAP_WINDOW_H
 #define KEYMAP_WINDOW_H