Add action callback function.

Storing the action callback as a member will allow us to execute actions
at any time, such as the start or end of a turn.
Added trait Regenerate.

Units with Regenerate regain all lost hp at the start of their turn.
Note that this doesn't resurrect any killed creatures, it just restores the top
one.
Added animation for Regeneration ability.
Notes and ideas for future development.
Added Sprite and Elf units.
Move up by 3 pixels to avoid covering size label.
Don't allow retaliation if a unit has Flying.

But do allow it if the defender has Flying too.
Handle animating a flying unit's movement.
Speed up Elf die animation so it looks smoother.
Fix behavior for flying units.

Allow the AI to correctly reason about them.
Allow the human player to fly and attack in one action.