Do not generate the uninstaller if the installation is not complete

Do not generate the uninstaller if the installation is not complete (packs have not been installed onto the filesystem).
Always ask for confirmation to exit the installation is not complete
Fix comment to specify that the quit method does not depend on if the installation is complete.

isCanClose() is only set the true after packs have been installed, otherwise is false.
I am unsure of the purpose the condition "!navigator.isNextEnabled() && !navigator.isPreviousEnabled()".
The above condition does not determine if the installation is complete.

Fix for http://jira.codehaus.org/browse/IZPACK-1063