Input/output API revamp

Problem

• Not all parts of the input and output APIs have clearly defined documentation, for example regarding memory allocation.
• Neither API exposes the parameters they can take, let alone any meta-information about them.
• Some of the output modules are still stuck on the very old API, which limits their usefulness.
• The lack of a well-thought out API for these is holding back the spinoff of these into libsigrokfile, which could be a very interesting project itself.

Ideas

• Clearly, we'll want to stick with what we've done for drivers where exposing parameters is concerned. That means:
  • An enum list like SR_CONF_* for the options, though not SR_CONF itself, as this would stop it from spinning off from libsigrok.
  • A meta structure like sr_config_info to expose short and long names and descriptions for all the keys.
  • GVariant as the mechanism to pass strongly typed values back and forth.