Bug 648767 — Return read-only views of Sets and Maps where possible

Bug 648767 — Return read-only views of Sets and Maps where possible

This prevents clients from messing up libfolks' state by modifying Sets or
Maps returned to them by libfolks when those Sets or Maps are just references
to the ones used internally by libfolks, as is the case with many
properties.

Closes: bgo#648767