shell, NM Model: Don't reset connection settings after failed apply

That would throw away uncommitted changes in the dialogs.

Reviewed-by: Stef Walter <stefw@redhat.com>
shell, NM Model: Export all address and route elements as strings

Because they are imported as strings.

Reviewed-by: Stef Walter <stefw@redhat.com>
shell, NM Model: Allow both netmask and prefix for IPv4 addresses and routes

Fixes #1181
Closes #1290
Reviewed-by: Stef Walter <stefw@redhat.com>