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
Well, yes, you're right, I'm starting from a different approach that is to have paths always "formed" the Unix way in Joomla, so developers can once and forever forget about related issues now and in future. The only attention they should use are:
- do not use DIRECTORY_SEPARATOR any more
- use `JPath::ux_clean()` liberally, whenever you have to deal with paths

Doing that, whatever string comparison between paths, **present and future**, would not create issues.