decoders: Fix incorrect 'outputs' fields. Only add items to 'outputs' if the respective PD actually has OUTPUT_PYTHON support implemented as of right now. Various decoders might get OUTPUT_PYTHON support later, but the 'outputs' field should reflect the current status.
all decoders: introduce a reset() method Move initialization code of protocol decoders from the constructor to a new reset() helper method. The libsigrokdecode backend could run this method several times to clear the decoder's internal state, before new data from another acquisition gets fed to decode() calls.