Get rid of DIRECTORY_SEPARATOR

This is a first tentative of standardizing on `/` as the directory separator (in Windows too...) and render the usage of DIRECTORY_SEPARATOR unneeded.

It **seems** to work but thorough testing is needed, especially under Windows.

I'm unsure of what I did with unit tests: they should be particularly screened and tested
@mbabker Reverted and created new ux_clean() function with '/' forced as the directory separator. In core I changed all calls to JPath:clean() to calls to JPath::ux_clean().

The function name is 'provisional': if we find something better... no problem adopting it!