IZPACK-1124

Addresses http://jira.codehaus.org/browse/IZPACK-1124?filter=16199 (Navigator and panel button mnemonics in GUI installer).

Proposed additions to the ButtonFactory class to add mnemonics (keyboard shortcuts) to userInputPanel and navigator buttons based on the first available letter of the button's caption text on any given panel.
- Navigator buttons (next, previous, quit, help) have their mnemonics set and reserved first so they do not collide with any other button mnemonics.
- Other panel buttons will have their shortcuts set according to the first available letter in their caption for that given panel. Example, if a single panel has two browse buttons, the first will have a shortcut of alt-b, and the second will have alt-r.
- Shortcuts are language dependent (ie. the french version of 'next' will have its shortcut be alt-s for 'suivant').