Using plain numbers make it cumbersome to grep for
setting types by priority.

The only downside is, that with the enum values it
is no longer obvious which value has higher or lower
priority.

Also, introduce NM_SETTING_PRIORITY_INVALID. This is what
_nm_setting_type_get_base_type_priority() returns. For the moment
it still has the same numerical value 0 as before. Later, that
shall be distinct from NM_SETTING_PRIORITY_CONNECTION.