NMConnection keeps a list (hash table) of all settings.

There are two lookup methods to find a setting in a connection:

  - nm_connection_get_setting() by GType
  - nm_connection_get_setting_by_name() by name

Note, that nm_connection_get_setting_by_name() first converts the
name to a GType, and then looks up the setting by GType.

But then, nm_connection_get_setting() would again convert the GType to
the type name, and hash the name. That is pointless, just index by GType
directly.

Maybe, using a hash table is anyway overkill because commonly there are
only a handful of settings in a connection. Regardless of that, change
the hashing.