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
Adding a new method instead of just supplying the second parameter as `/` for every `JPath::clean()` call was simpler because...

I'm still not convinced this needs to be handled globally in every instance of path handling, which is still what this PR is trying to do right now (understand it's essentially proof of concept, but it's overkill).  In truth, I'd rather see several smaller PRs targeted specifically at areas that need to be addressed.  Why not start with the path constants and the handling of the favicon since the former seems to have caused problems with the PR changing those and the latter has been a known issue for quite some time.  Then either set up a Windows system to run the unit tests to see where there are path related errors (Hannes may be able to help with this, I know he runs the tests on occasion and has Windows) and branch out or just use the CMS and find problem areas related to path handling.  In the PHP code I know of that interacts with the local filesystem, I'm not aware of any that just blanket converts everything to using one separator style, so we should not need to either.