EclipseHelper: handle spaces in the directory path

Even if the relative paths in class path cannot have spaces, the
directories making up the class path can very well contain spaces
somewhere along the lines.

By using the FileUtils to convert from a file: URL to a File, we
handle spaces and all kinds of funny characters in the directory
names at the same time.

Pointed out by Christian Dietz.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Test that the EclipseHelper can handle spaces in directory names

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