[Feat] Add the ability to polymorph an actor by dropping an actor on another actor sheet

[Pjb518]

Dropping actor A onto the sheet for actor B should change actor B's statistics in some way, as defined by the type of polymorph being performed. Initially, we should support A5e Wildshape and the A5e Polymorph and Shapechange spells. Later on, we can also support 5e Wildshaping.

On drop, a dialog will need to be presented to the user so that they can choose a polymorph mode to use.

Importantly, this needs to be a reversible process. It needs to be possible to convert an actor back to their original form at any time.