WIP: PoC implementation for the PCA9685

[mzanetti]

nymea-plugins pull request checklist:

• [ ] Make sure the pull request's title is of format "Plugin name: Add support for xyz" or "New plugin: Plugin name"

• [ ] Did you test the changes on hardware, if not (e.g. absence of required hardware), please mention a person to confirm it has been tested.

• [ ] Did you update the plugin's README.md accordingly?

• [ ] Did you update translations (cd builddir && make lupdate)?

• [ ] If you added a new plugin, should it be added to nymea-plugins-all or nymea-plugins-maker?

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[nymea-jenkins]

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