GUI should only write forward slashes to setup files

[chrisdembia]

@jimmyDunne and I just chatted and we think that on Windows, the GUI often writes backslashes to setup files. This causes setup files to not be portable to Mac/Linux. @jimmyDunne suggested that even on Windows, the GUI should use forward slashes.

[aymanhab]

A file or path property should take care of that and other issues including drive name, absolute/relative paths etc.