Allow Multiple Topics via "MQTTSubscribe" setting.

[ghormann]

Currently, the MQTTSubscribe setting can be used to subscribe to a single MQTT PATH. (The Path can include wild cards). With this enhancement, the user would be able to subscribe to multiple paths.

Proposed implementation is to keep a single Settings variable, but use ";;;" as a separator. (Semi-colin is a valid character in a topic name, but it is unlikely to see three together). -- Proposing this implementation as we don't have a setting type that supports list.