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
The way you handled it puts the burden of updating code on developers.  If the intent is to standardize Joomla's internal handling of the separators, then the API endpoints should be the ones doing the conversions.  For example, all of your changes to `JLoader::register()` calls should be reverted and that method instead updated to ensure all paths that it receives are using only the `/` separator.

Obviously, not everything is an API endpoint (like component internals), nor does everything need to always use the `/` separator.  Like I noted before, if we aren't doing any sort of comparison (like in the `JLoader::register()` calls or the `JLog::add()` calls), we're probably OK leaving things as is for now.

Truthfully, this would be an instance where it'd be great to have a Windows server hooked up to our continuous integration services.  While the unit tests are all passing now (Linux based system driving it), I have a feeling that there are some failures introduced in the Windows environment and we unfortunately don't have a way to catch them.