Safer file removal.

When the Symfony FileSystem component was introduced, a slight caveat
was overseen; when FileSystem#remove() receives a directory, it will
remove that directory... probably for convenience reasons.

The issue with this is that sometimes we concatenate whole paths and the
delete them... SpoonFile::delete() would silently not delete anything if
the file was not found, but FileSystem does not behave in the same way;
take the blog edit action in the backend for instance, due to flawed
logic, following code would delete the whole blog/source directory:

    if($this->frm->getField('image')->isFilled())
    {
        $fs->remove($imagePath . '/source/' . $this->record['image']);
        // ...
    }

In case we have added an article without an image, and then later on, we
upload an image => $this->record['image'] will be empty causing the
whole source/ directory to be emptied... which is _not_ what we want.

I have placed checks (is_file($filename)) all over where I deemed
necessary to more adhere to the previous behaviour of
SpoonFile::delete() so that this will not happen in the base
distribution of fork.
Change var name: $fileName was confusing with argument $filename.
Merge pull request #560 from forkcms/safer-file-remove

Safer file removal.