libnm-core: change connection hash tables to variants in API

libnm-core: change connection hash tables to variants in API

In preparation for porting to GDBus, make nm_connection_to_dbus(),
etc, represent connections as GVariants of type 'a{sa{sv}}' rather
than as GHashTables-of-GHashTables-of-GValues.

This means we're constantly converting back and forth internally, but
this is just a stepping stone on the way to the full GDBus port, and
all of that code will go away again later.