struct sr_samplerates is now obsolete

struct sr_samplerates is now obsolete

sr_config_get() of SR_CONF_SAMPLERATE now returns a GVARIANT_TYPE_DICTIONARY.
This dictionary contains a single key-value pair. Possible values for the key:

 - "samplerates": the value is an array of GVARIANT_TYPE_UINT64 representing
   all valid samplerates.
 - "samplerate-steps": the value is an array of GVARIANT_TYPE_UINT64 with
   exactly three members, which represent the lowest samplerate, highest
   samplerate, and the minimum step, respectively.