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.

Incidentally, this also fixes *another* problem Christian pointed
out: on Windows, the file: URLs cannot be turned into proper
paths by stripping the "file:" prefix because the URLs start with
a slash to indicate absolute paths, then continue with the DOS
style drive letter and a colon because that is what Windows calls
an absolute path.

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>
Merge branch 'with-spaces'

This topic branch fixes the problems reported by Christian Dietz that
prevented the EclipseHelper to work properly on Windows, or when the
directories' absolute paths contained spaces.

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