Improve accessibility of file browser items

More info in the commit message as usual.

I guess the main point of discussion here is where the appropriate code belongs - whether or not it is right to set the `representedObject` of `OFBPathInfoCell` to the relevant FSItem and use this info
rmation inside the `OFBPathInfoCell` accessibility support (this is the way I did it as I felt `OFBPathInfoCell` should know its accessibility since it could be used in more than one place, like File C
hooser, which, however, does not seem to reflect this accessibility support as it has its own logic to determine file attributes), or whether to set the accessibility attributes as override values in `
[FSOutlineViewDelegate willDisplayCell:...]`.

These patches are released to public domain by my employer, BRAILCOM,o.p.s.