Cache get_documents_path value

As the xdg value will not change in the session,
we can cache the value and avoid executing xdg-user-dir
every time.
Assign the right mountpoint to files located in Documents directory
Don't remove the extension on external files - Fixes #2949

The file is renamed but the title lost the extension.
More information in the ticket. This change solves the problem
at save the metadata, but there are a remaining issue, the listview
is not correctly updated.
Save/Load metadata for files in subdirectories in Documents and devices

This was not implemented before. A directory is created as needed
inside .Sugar-Metadata, according to the file path.
Fix delete metadata of files in subdirectories
Notify the view about changes in external devices

Now modify the object title, or setting favorite works
in external devices, because the view is notified of the change.