Avoid absolute paths in "On This Computer" source groups.

Avoid absolute paths in "On This Computer" source groups.

A base URI of "local:" is sufficient to identify the "On This Computer"
source group.  No other source group uses that URI scheme, and it avoids
an entire class of portability problems that come with using an absolute
"file:" URI to point to user-specific data:

- What if the user's home directory is renamed?
- What if settings are moved to a new host under a different user name?
- What if settings are copied to another user account on the same host?
- Migrating to XDG base dirs is otherwise a giant PITA.