Fix MoveMenu cursor memory bug

The cursor position wasn't being saved when the player selected a move
and then cancelled target selection.
Fix level regression in SkillUsable

This should have been fixed a while ago. The level of learned skills was
being calculated incorrectly due to earlier changes to the stat system
(due to laziness, skills had been using a Stat object internally to
represent their level).
Partial implementation of Ghost status

Incoming attacks miss if attacker isn't Ghostified; still need to
implement outgoing attack handling