Fix FileUtils#urlToFile() on Windows

Under certain circumstances, on Windows, URLs are translated into
strings of the form "file:C:/..." (which the URL constructor does not
like at all) instead of "file:/C:/..." (which the URL constructor
does like).

Let's be lenient on Windows and just accept even the first form.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add utility class for dealing with platform issues

In particular, it has some classic OS detection methods.
Accelerator: strengthen TODO to FIXME
Use PlatformUtils methods where applicable
Add a regression test for urlToFile(directory)

Java on Windows really has problems in that department, omitting a slash
when called via toURI().toURL() (but not when the deprecated
File#toURL() method is called...).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>