Encode the path part of folder URIs.

Wasn't sure if this was necessary, but it -is- in order to handle the
local Junk and Trash vfolder names correctly:

    .#evolution/Junk
    .#evolution/Trash

If we don't escape the path and we feed camel_url_new() something like
"folder://local/.#evolution/Trash", it's gonna think the path is '.' and
the rest of it's a fragment.