[IZPACK-1151] - Shortcut Improvements

This is to address: https://jira.codehaus.org/browse/IZPACK-1151

1. Desktop shortcuts are now independent of start menu shortcuts.
2. Startup shortcuts are now independent of start menu shortcuts.
3. Data placed in automatic installation file has been updated accordingly to support independence of desktop and startup shortcuts.
4. If the shortcut specification contains the attribute "application" but does not contain the attribute "programGroup", then we do not allow to select some sub-directory since it is always placed on top of the application hierarchy. Also a "(Default)" option has been added to act as though no option was selected, this is mostly for UI purposes.
5. Refactoring was done based on point #5 of the linked issue.