When not hovering over an enemy, try to focus the last hit enemy

This is essential for mouse-less systems, since MenuEnemy was only
displayed when the mouse cursor was over an enemy. It's also useful for
systems that *do* use a mouse, since the player might not be clicking
directly on the enemy, like when leading a projectile shot.
Reorganize some lines for readability
Merge pull request #708 from dorkster/menu_enemy

When not hovering over an enemy, try to focus the last hit enemy