Add Slideshow attributes to preferences

[speechles]

both slideshow_duration and slideshow_overlay presently are stored in the registry. This adds a way to configure the two.

[PhillipAnd]

This is incorrect - the registry key should be slideshow_period, not slideshow_duration

But this is definitely an essential feature that needs to be added.

[speechles]

This likely will never be added officially, look how stale the request is. But nice catch, the blue neon client has adopted this change. Thx PhillipAnd