Add hex highlight for ranged attack target.
Added attack arrows.

Assume that multi-frame images are sized such that each frame fits in a
hex.
Copied files from sdltest project for pathfinding.
Add array index representation for a hex.

This should help with pathfinding.
Have a unit keep track of which hex it's in.
Copy Pathfinder from sdltest project.
Starting to use pathfinding for valid moves.
Fixed subtle miscalculation in direction().
Be smarter about computing possible actions.
Compute possible action at mouse coordinates.
Include the number of moves for each unit.
Add handling for ranged attacks.