Merge DecoderStack into DecodeSignal

Several changes make up this commit, which unfortunately
can't be separated:

1) Move decoder stack management from DecoderStack to
DecodeSignal, thereby making DecoderStack unnecessary

2) Change the decoder stack from std::list to an
std::vector for direct decoder access

3) Introduce logic_mux_thread which will take care
of muxing the individual SignalBases' logic data into
(cached) logic data that libsigrokdecode expects.
This is necessary as we can no longer do simple bit
mapping within a single logic data segment's logic
data as we now may feed from multiple logic data
segments at once

4) Refactored the creation of decode traces, making
it more streamlined and flexible while simplifying
the class interface

5) Refactored the auto-assignment of channels

6) Refactored is_decode_signal()

7) Reworked decode signal save/restore, allowing
proper handling and with the decoder stack now
being part of the signal, easier save/restore of
the stack and its settings