Fixes to the directory filter list

The older commit fixes a clear bug: when the initial part of the path is repeated further, we only want to replace the first instance (this is especially crucial when said part is '/'...)

The second one is more open to discussion, and I wonder if a better solution could be found: chrome and at least some versions of IE seem to ignore the margin-left specs on <option> elements, so I replaced them by multiple nbsps, which is not too nice. 

Both changes issued from this issue:
https://github.com/koniu/recoll-webui/issues/23#issuecomment-33959570