Reduce calls to Meural API

[GuySie]

We've already severely cut back on calls to the Meural API where possible, e.g. no calls are made when the device is sleeping, and all calls to change item or playlist are made locally if possible.

But there are options - like image duration and shuffle - that are only exposed via the device options in the Meural API, and not exposed in the local interface at all. The integration currently polls the Meural API every 10 seconds to update this information.

We should find a way to reduce these calls to the Meural API too.