sharing: Fix deactivation of services when changing network

When switching between networks that both allow sharing in general,
we need to check the enabled connections for each service and
possibly stop it.

https://bugzilla.gnome.org/show_bug.cgi?id=732218
sharing: set_properties() was unused when NM is there
sharing: use UUIDs instead of IDs for NM connection identification

Despite the name, the connection ID is a human readable string
and is not guaranteed to be unique.

https://bugzilla.gnome.org/show_bug.cgi?id=732218
sharing: Expand API to expose network names

We were using UUIDs which aren't human-readable as per commit
bc3be2b760963688f15d28a853cd99954594e979
sharing: Remove useless set_properties()

Added in commit 5cfb8fd711bb5b5b93f519cf2c1248d251861f5a but
of no use, as those values are already set in _init()