Quickfix to make the XWiki Installer work on Windows7

This quickfix consists in setting the default install dir to be c:\XWiki Enteprise instead of Program Files (since Program Files is read only on Windows7 and thus XWiki will fail to write to it)

See XE-1051 for the proper fix which is available only for 4.2M1+. We didn't backport it since it contains too many changes and was thus too risky for the 4.1 final release.