Various Python decoder infrastructure improvements.

Various Python decoder infrastructure improvements.

 - Introduce 'struct sigrokdecode_decoder'.

 - Decoders are now handled via two C functions:
   - sigrokdecode_load_decoder(): Fills a 'struct sigrokdecode_decoder'.
   - sigrokdecode_run_decoder(): Runs a decoder function.

 - There are now two decoder API functions a script needs to implement:
   - register(): Returns a Python dict with certain metadata.
   - decode(): Runs the actual decoder code.

 - libsigrokdecode: Add and use some more #defines for errors:
   - SIGROKDECODE_ERR_ARGS
   - SIGROKDECODE_ERR_PYTHON

 - Various other smaller Python decode script infrastructure issues.