libnm-core: rename NMConnection to/from_hash methods

libnm-core: rename NMConnection to/from_hash methods

Rename nm_connection_to_hash() to nm_connection_to_dbus(), and
nm_connection_new_from_hash() to nm_connection_new_from_dbus(). In
addition to clarifying that this is specifically the D-Bus
serialization format, these names will also work better in the
GDBus-based future where the serialization format is GVariant, not
GHashTable.

Also, move NMSettingHashFlags to nm-connection.h, and rename it
NMConnectionSerializationFlags.