Investigate overriding core AppV2 actions

[JPMeehan]

ActorSheetV2 implements three native actions - configurePrototypeToken, showPortraitArtwork, and showTokenArtwork, which are handled by static private functions but can be overridden by ActorSheetV2 subclasses. Right now, these actions are not represented so they can be highlighted for collision or overrides. Other sheet subclasses will also have actions - calling these actions is done by the data-action property, so normally type hints aren't super relevant but it is an interface people may want to use to modify behavior.