Feature: expose additional effect settings

[rbergen]

Recently, the ability was added for effects to expose settings that can be changed via the API. A few basic settings have been exposed by LEDStripEffect; the PatternSubscribers effect adds two more for itself.

Obviously, there are more effects for which it makes sense to expose properties as settings that can be changed via the device API and (later) the device web UI.

When working on this:

• The REST_API.md file contains information about the effect settings API endpoints.
• The aforementioned PatternSubscribers effect provides an example of an effect extending the basic settings exposed by LEDStripEffect.
• LEDStripEffect includes a number of helper functions for changing effect settings of "known types". Known types are themselves defined by the SettingSpec struct in types.h.