Load default directory from resources if there is no variable specified

The current version of 4.3 broke the ability to specify default install directories using the TargetDir.os.xxx resource files as described in http://izpack.org/documentation/panels.html#targetpanel.

This fixes the problem by reading in and using the resource files if no TargetDir.os.xxx variable is specifically specified.

Edit: Not sure why it's merging all my commits into this pull request.  I wanted to make separate requests and one of them I don't even want to push.