BaseIO::makeRelativePath(): create canonical filename before making the path relative

This makes sure that a common prefix can actually be detected even if basePath ends with a "/".
The previous version would fail in that case.
However, the file paths have to exists on the filesystem or else an exception is thrown.

git-svn-id: http://svn.code.sf.net/p/simox/code/trunk@558 042f3d55-54a8-47e9-b7fb-15903f145c44