Add new power category: "Channel"

[LockerM]

From discussion in PR #169, add a new type of Powers in the Actions category called Channel that is distinct from Minor/Major.

A channel power takes a Major action to initiate but has a channeling time, the amount of time a character needs to spend focusing exclusively on the Power to produce the effect.

Using a Minor or Major action while channeling ends the channeling time and must be restarted from the beginning.

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Undetermined - to be resolved: how does taking damage affect channeling? Does it force a save or does it automatically stop a character from channeling?