LibPinMAME: add pinball plugin support

[vbousquet]

This PR is a follow up from #303 which purposes were to:

• allow VPX to perform correct DMD rendering (needs unprocessed DMD frames),
• allow VPX's plugin to perform coloring (needs unprocessed DMD frames),
• allow VPX to include a backglass renderer with support for fading lamps & flashers (could be done with existing interface but at the price of lots of data exchange and likely some API bloat),
• allow VPX to support modulated segments,
• allow VPX to add support for video output (MrGame machines, ...).

Since this draft PR was created, a plugin architecture was prototyped on the VPX side. This plugin architecture serves similar purpose but with a broader application range. It is organized around a simple message bus, allowing better collaboration between components. This PR replace the stateblock approach by the message bus approach used by the plugin system. Thanks to the plugin design, this PR serves the original aims with the following benefits:

• simpler and cleaner code,
• no more duplication between stateblock and plugin messages
• more future proof, as it allows sharing new data by adding new messages if they can't be fitted in the initial API,
• a bit better data definitions, also allowing easier UX integration.

This is a draft PR since the plugin design is not stable yet and will likely evolve some more, as it developped by designing the API then validating against 2 or 3 real world use cases. This validation phase is not yet finished.